TECHNICAL FIELD The present invention relates to a separator of an electrochemical device such as a lithium secondary battery, a manufacturing method thereof and an electrochemical device containing the same, and more particularly to an organic/inorganic composite separator in which a porous active layer is coated with a mixture of an inorganic material and a polymer onto a surface of a porous substrate, a manufacturing method thereof and an electrochemical device containing the same. BACKGROUND ART Recently, there has been an increasing interest in energy storage technology. Batteries have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, the research and development into a novel electrode and a novel battery that can improve capacity density and specific energy have been made intensively in the field of the secondary batteries. Among currently used secondary batteries, lithium secondary batteries developed in early 1990's have a higher drive voltage and a much higher energy density than those of conventional batteries using an aqueous electrolyte solution (such as Ni-MH batteries, Ni-Cd batteries, H2SO4-Pb batteries, etc). For these reasons, the lithium secondary batteries have been advantageously used. However, such a lithium secondary battery has disadvantages in that organic electrolytes used therein may cause safety-related problems resulting in ignition and explosion of the batteries and that processes for manufacturing such a battery are complicated. Recently, lithium-ion polymer batteries have been considered as one of the next-generation batteries since the above disadvantages of the lithium ion batteries were solved. However, the lithium-ion polymer batteries have a relatively lower battery capacity than those of the lithium ion batteries and an insufficient discharging capacity in low temperature, and therefore these disadvantages of the lithium-ion polymer batteries remain to be urgently solved. Such a battery has been produced from many companies, and the battery stability has different phases in the lithium-ion polymer batteries. Accordingly, it is important to evaluate and ensure the stability of the lithium-ion polymer batteries. First of all, it should be considered that errors in operation of the batteries should not cause damage to users. For this purpose, the Safety and Regulation strictly regulate the ignition and the explosion in the batteries. In order to solve the above battery safety-related problem, there has been proposed an organic/inorganic composite separator having a porous active layer formed by coating at least one surface of a porous substrate having pores with a mixture of inorganic particles and a binder polymer. The porous active layer formed of this conventional organic/inorganic composite separator shows homogeneous composition morphology toward a thickness direction, as shown in FIG. 2B and FIG. 3B. However, if the electrochemical device is assembled with the organic/inorganic composite separator, it has disadvantages in that inorganic particles in the porous active layer are detached and a lamination characteristic toward electrodes is deteriorated during a winding process, etc. If a content of a binder polymer in the porous active layer is increased so as to solve the above disadvantages, characteristics such as the peeling and scratch resistances, the lamination characteristic toward electrodes, etc. in the assembly process of the electrochemical device may be rather improved, but porosities in the porous active layer are decreased since the inorganic particles are present in a relatively lower content, resulting in deterioration in performances of the electrochemical device, and the safety of the separator is also reduced due to the introduction of the porous active layer. DISCLOSURE OF INVENTION The present invention is designed to solve the problems of the prior art, and therefore the first object of the invention is to provide an organic/inorganic composite separator capable of improving characteristics in an assembly process of an electrochemical device without any increase in the content of a binder polymer so that a porous active layer with which at least on surface of the organic/inorganic composite separator is coated can maintain sufficient porosities, a manufacturing method thereof and an electrochemical device containing the same. The present invention is designed to solve the problems of the prior art, and therefore the second object of the invention is to provide a method for manufacturing an organic/inorganic composite separator having characteristics as described in the first object by undergoing only a single coating process. In order to accomplish the first object, the present invention provides an organic/inorganic composite separator including: (a) a porous substrate having pores; and (b) a porous active layer containing a mixture of inorganic particles and a binder polymer with which at least one surface of the porous substrate is coated, wherein the porous active layer shows heterogeneity of composition morphology toward a thickness direction in which a content ratio of the binder polymer/inorganic particles present in a surface region of the porous active layer is higher than that of the binder polymer/inorganic particles present inside the porous active layer, a manufacturing method thereof and an electrochemical device containing the organic/inorganic composite separator. The above-mentioned organic/inorganic composite separator of the present invention may enhance peeling and scratch resistances of the porous active layer and improve a lamination characteristic toward electrodes by introducing a porous active layer onto a porous substrate having pores, the porous active layer having heterogeneity of morphology toward a thickness direction in which a content ratio of the binder polymer/inorganic particles present in a surface layer is higher than that of the binder polymer/inorganic particles present inside the surface layer. Accordingly, stability and performances of the battery can be all improved together since the detachment of inorganic particles from the porous active layer may be reduced in the assembly process of the electrochemical device. In the organic/inorganic composite separator of the present invention, a first binder polymer is preferably used as the binder polymer, the first binder polymer containing together at least one functional group selected from the group consisting of carboxy, maleic anhydride and hydroxy; and at least one functional group selected from the group consisting of cyano and acrylate. Such a first binder polymer includes cyanocthylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, etc. In the organic/inorganic composite separator of the present invention, a second binder polymer having a solubility parameter of 17 to 27 MPa1/2 is preferably used as the binder polymer together with the above-mentioned binder polymer in the aspect of electrochemical stability of the porous coating layer. Such a second binder polymer includes polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyimide, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, etc. In order to accomplish the second object, the present invention provides a method for manufacturing an organic/inorganic composite separator including a porous active layer, the method including: (SI) preparing a solution of a first binder polymer containing together at least one functional group selected from the group consisting of carboxy, maleic anhydride and hydroxy; and at least one functional group selected from the group consisting of cyano and acrylate; (S2) adding inorganic particles to the solution of the first binder polymer and dispersing the inorganic particles in the solution of the first binder polymer; (S3) coating the solution of the first binder polymer having inorganic particles dispersed therein with a film and drying the coated film, wherein the porous active layer shows heterogeneity of morphology toward a thickness direction in which a content ratio of the first binder polymer/inorganic particles present in a surface region of the porous active layer is higher than that of the first binder polymer/inorganic particles present inside the porous active layer. In the method for manufacturing an organic/inorganic composite separator of the present invention, the second binder polymer having a solubility parameter of 17 to 27 MPa1/2 is preferably further dissolved in the solution of the first binder polymer in the aspect of electrochemical stability of the porous coating layer. BRIEF DESCRIPTION OF THE DRAWINGS These and other features, aspects, and advantages of preferred embodiments of the present invention will be more fully described in the following detailed description, taken accompanying drawings. In the drawings: FIG. 1 is a diagram showing a cross-sectional view of an organic/inorganic composite separator according to the present invention and a schematic view of an active layer having heterogeneity of morphology toward a thickness direction. FIG. 2 is a photograph, taken by a scanning electron microscope (SEM), showing the organic/inorganic composite separator. Here, FIG. 2A is a magnified photograph showing a surface of the porous active layer having heterogeneity of morphology toward a thickness direction prepared in Example 1, and FIG. 2B is a magnified photograph showing a surface of a conventional porous active layer. FIG. 3 is a photograph, taken by a scanning electron microscope (SEM), showing the organic/inorganic composite separator. Here, FIG. 3A is a magnified photograph showing a surface of the porous active layer having heterogeneity of morphology toward a thickness direction prepared in Example 1, and FIG. 3B is a magnified photograph showing a surface of a conventional porous active layer. FIG. 4A is a photograph showing a peeling characteristic in a surface of an organic/inorganic composite separator prepared in Example 1, wherein the organic/inorganic composite separator has a porous active layer formed therein, the porous active layer having heterogeneity of morphology toward a thickness direction, and FIG. 4B is a photograph showing a peeling characteristic in a surface of a conventional organic/inorganic composite separator, wherein the organic/inorganic composite separator has a composite coating layer formed therein, the composite coating layer being composed of an inorganic material and a polymer. FIG. 5 is a photograph taken after an organic/inorganic composite separator prepared in Example 1 is laminated into an electrode, wherein the organic/inorganic composite separator has a porous active layer formed therein, the porous active layer having heterogeneity of morphology toward a thickness direction. BEST MODES FOR CARRYING OUT THE INVENTION Hereinafter, preferred embodiments of the present invention will be described in detail referring to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the, best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention. Unlike conventional composite separators, such as a polyolefin separator having a porous active layer, wherein the porous active layer has homogeneous morphology toward a thickness direction is simply formed on a porous substrate, the present invention provides an organic/inorganic composite separator including a porous active layer having heterogeneity of composition morphology toward a thickness direction in which a content ratio of the binder polymer/inorganic particles present in a surface region of the porous active layer is higher than that of the binder polymer/inorganic particles present inside the porous active layer. 1) The organic/inorganic composite separator of the present invention includes a porous substrate 1 and a porous active layer 3 formed in at least one surface of the porous substrate 1, wherein the porous active layer 3 includes a polymer 5 and inorganic particles 9, the content ratios of the a polymer 5 and the inorganic particles 9 are varied toward a thickness direction, as shown in FIG. 1. Accordingly, the organic/inorganic composite separator has increased resistances to external stimuli such as a peeling resistance, a scratch resistance, etc. and an improved lamination characteristic toward electrodes due to adhesion characteristic of the polymer present in a large amount in the surface of the active layer. Therefore, the organic/inorganic composite separator of the present invention may exhibit very excellent characteristics in the assembly process of a battery such a winding process, a lamination process, etc. (see FIG. 4A). Also, the organic/inorganic composite separator of the present invention may have excellent ion conductivity since the heterogeneity of morphology toward a thickness direction enables the porosity of the active layer to become increased from its surface to its inside, thereby resulting in improved battery performances. 2) Also, the complete internal short-circuit between the electrodes is hard to occur because of the presence of an organic/inorganic composite porous active layer even if the porous substrate is ruptured inside of the battery, and the short-circuited area is not enlarged any more although a short-circuited phenomenon appears in the battery, resulting in improvement in safety of the battery. In the application of the present invention, the expression "heterogeneity of morphology toward a thickness direction in which a content ratio of binder polymer/inorganic particles present in a surface region of a porous active layer is higher than that of binder polymer/inorganic particles present inside the porous active layer" should be understood to include all aspects if the organic/inorganic composite separator of the present invention is formed so that a content ratio of binder polymer/inorganic particles present in a surface of a porous active layer is higher than that of binder polymer/inorganic particles present beneath (inside) the surface of the porous active layer. For example, by the expression, it is meant that the organic/inorganic composite separator of the present invention includes all porous active layers including a porous active layer formed so that the content ratio of the binder polymer/inorganic particles is linearly decreased toward a direction from a surface of the porous active layer to the porous substrate; a porous active layer formed so that the content ratio of the binder polymer/inorganic particles is non-linearly decreased toward a direction from a surface of the porous active layer to the porous substrate; a porous active layer formed so that the content ratio of the binder polymer/inorganic particles is non-continuously decreased toward a direction from a surface of the porous active layer to the porous substrate, etc. In the application of the present invention, the content ratio of the binder polymer/inorganic particles is also determined on the basis of the entire surface region of the porous active layer since the binder resin present in the surface region of the porous active layer may not be partially homogenously mixed with the inorganic particles. One of major components in the organic/inorganic composite separator according to the present invention are inorganic particles generally used in the art. The inorganic particles are the major components used to manufacture a final organic/inorganic composite separator, and serve to form micropores due to the presence of interstitial volumes among the inorganic particles. Also, the inorganic particles also serve as a kind of a spacer capable of maintaining a physical shape of a coating layer. The inorganic particles used in the organic/inorganic composite separator of the present invention are stable in the electrochemical aspect, but the present invention is not particularly limited thereto. That is to say, the inorganic particles, which may be used in the present invention, is not limited if oxidation and/or reduction reactions do not take place within the operation voltage range (for example, 0~5V in a Li/Li+ battery) of a battery to be applied. In particular, the inorganic particles having ion conductivity may improve performances of the organic/inorganic composite separator by enhancing ion conductivity in the electrochemical device. Further, when inorganic particles having high dielectric constant are used, they can contribute to increasing the dissociation degree of an electrolyte salt in a liquid electrolyte, such as a lithium salt, thereby improving the ion conductivity of the electrolyte. For these above reasons, the inorganic particles preferably include inorganic particles having a high dielectric constant of 5 or more, and more preferably 10 or more, inorganic particles having lithium conductivity or mixtures thereof. A non-limiting example of the inorganic particles having a dielectric constant of 5 or more include BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb,.xLaxZri.yTiyO3 (PLZT), PB(Mg3Nb2/3)O3-PbTiO3 (PMN-PT), hafnia (HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, Y2O3, A12O3, TiO2, SiC or mixtures thereof. In particular, the above-described inorganic particles, for example BaTiO3, Pb (Zr,Ti)O3 (PZT), Pb,_xLaxZr,_yTiyO3 (PLZT), PB (Mg3Nb2/3)O3-PbTiO3 (PMN-PT) and hafnia (HfO2), has a high dielectric constant of 100 or more. The inorganic particles also have piezoelectricity so that an electric potential between both surfaces can be generated in the presence of the generated charges when pressure is applied over a critical level. Therefore, the inorganic particles can prevent internal short circuit between both electrodes, thereby contributing to improving the safety of a battery. Additionally, when such inorganic particles having a high dielectric constant are mixed with inorganic particles having lithium ion conductivity, synergic effects may be obtained. As used herein, "inorganic particles having lithium ion conductivity" are referred than 10 /an, the resultant porous active layer has an increasing thickness, resulting in degradation in mechanical properties. Furthermore, such excessively large pores may increase a possibility of generating internal short circuit during repeated charge/discharge cycles. As one of the major components in the organic/inorganic composite separator having heterogeneity of morphology toward a thickness direction according to the present invention, a first binder polymer is preferably used as the binder polymer, the first binder polymer including together at least one functional group selected from the group consisting of carboxy, maleic anhydride and hydroxy; and at least one functional group selected from the group consisting of cyano and acrylate. More preferably, first binder polymers containing a hydroxy group and a cyano group together, such as cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, are used alone or in combinations thereof. If a coating solution using the first binder polymer having two predetermined functional groups is used herein, an organic/inorganic composite separator having heterogeneity of morphology toward a thickness direction is easily formed by only one coating by means of control of phase inversion, and a cohesion force among the inorganic particles, an adhesion force between the porous active layer and the porous substrate and a lamination characteristic toward electrodes are further improved. In the manufacturing process of a battery, one of the very important characteristics is particularly the lamination to electrodes of the porous active layer formed in the organic/inorganic composite separator. The lamination characteristic toward electrodes is evaluated by measuring an adhesion force between separators, to as inorganic particles containing lithium ions and having a capability of transferring lithium ions without storing lithium. The inorganic particles having lithium ion conductivity can conduct and move lithium ions due to defects in their particle structure, and thus can improve lithium ion conductivity and contribute to improving battery performance. A non-limiting example of such inorganic particles having lithium ion conductivity includes: lithium phosphate (Li3PO4), lithium titanium phosphate (LixTiy (PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (LixAlyTiz (PO4)3, 0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP)xOy type glass (0 < x < 4,0 < y < 13) such as 14Li2O-9Al2O3-38TiO2-39P2O5, lithium lanthanum titanate (LixLayTiO3, 0 < x < 2,0 < y < 3), lithium germanium thiophosphate (LixGeyPzSw, 04). lithium titanium phosphate (LixTiy (PO4)3, 0 < x < 2, 0 < y < 3), lithium alumium titanium phosphate (LixAlyTi7. (PO4)3, 0