ORGANIC/INORGANIC COMPOSITE SEPARATOR AND ELECTROCHEMICAL DEVICE CONTAINING THE SAME TECHNICAL FIELD The present invention relates to a separator of an electrochemical device such as a lithium secondary battery and an electrochemical device containing the same. More particularly, the present invention relates to an organic/inorganic composite separator having a porous coating layer formed with a mixture of inorganic particles and polymer on a surface of a porous substrate, 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 a liquid electrolyte solution such as Ni-MH batteries, Ni-Cd batteries, and H2SO4-Pb batteries. 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 such as 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 are 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 electrochemical devices have been produced from many companies, and the battery stability has different phases in the electrochemical devices. Accordingly, it is important to evaluate and ensure the stability of the electrochemical batteries. First of all, it should be considered that errors in operation of the electrochemical device should not cause damage to users. For this purpose, the Safety Regulation strictly regulates ignition and explosion in the electrochemical devices. In the stability characteristics of the electrochemical device, overheating of the electrochemical device may cause thermal runaway, and explosion may occur when a separator is pierced. In particular, a polyolefin porous substrate commonly 2 used as a separator of an electrochemical device shows extreme thermal shrinking behavior at a temperature of 100°C or above due to the features of its material and its manufacturing process such as elongation, so there may occur an electric short circuit between cathode and anode. In order to solve the above safety-related problems of the electrochemical device, Korean Laid-open Patent Publication No. 10-2006-72065 and No. 10-2007-231 disclose an organic/inorganic composite separator 10 having a porous coating layer formed by coating at least one surface of a porous substrate 1 having many pores with a mixture of inorganic particles 3 and a binder polymer 5 (see FIG. 1). In the organic/inorganic composite separator, the inorganic particles 3 in the porous coating layer formed on the porous substrate 1 act as a kind of spacer that keeps a physical shape of the porous coating layer, so the inorganic particles restrain thermal shrinkage of the porous substrate when the electrochemical device is overheated. In addition, interstitial volumes exist among the inorganic particles, thereby forming fine pores. As mentioned above, at least a certain amount of inorganic particles should be contained such that the porous coating layer formed on the organic/inorganic composite separator may restrain thermal shrinkage of the porous substrate. However, as the content of inorganic particles is increased, a content of binder polymer is relatively decreased, which may cause inorganic particles to be extracted from the porous coating layer due to the stress generated in an assembly process of an electrochemical device such as winding. The extracted inorganic particles act as a local defect of the electrochemical device, thereby giving a bad influence on the stability of the electrochemical device. DISCLOSURE OF INVENTION Technical Problem The present invention is designed to solve the problems of the prior art, and therefore an object of the invention is to provide an organic/inorganic composite separator, which has good thermal stability so as to restrain an electric short circuit between cathode and anode even when an electrochemical device is overheated, and also which may improve stability of the electrochemical device by preventing extraction of inorganic particles in a porous coating layer formed on a porous substrate during an assembly process of the electrochemical device. Technical Solution In order to accomplish the first object, the present invention provides an organic/inorganic composite separator, which includes a porous substrate having a plurality of pores; and a porous coating layer formed on at least one surface of the porous substrate with a plurality of inorganic particles and a binder polymer, wherein the binder polymer is a copolymer including: (a) a first monomer unit having a 3 contact angle to a water drop in the range from 0° to 49°; and (b) a second monomer unit having a contact angle to a water drop in the range from 50° to 130°. The organic/inorganic composite separator according to the present invention has an excellent peeling resistance, so it may prevent inorganic particles in the porous coating layer from being extracted during an assembling process of an electrochemical device. Also, though an electrochemical device is overheated, thermal shrinkage is restrained, which allows to prevent any electric short circuit between cathode and anode. Accordingly, the safety of the electrochemical deice is greatly improved. In the organic/inorganic composite separator according to the present invention, the first monomer unit is preferably a monomer unit having at least one functional group selected from the group consisting of OH, COOH, MAH (maleic anhydride) and SO3H, and the second monomer unit is preferably a monomer unit having at least one functional group selected from the group consisting of F, Cl, CN, acrylate, acetate and ester. The copolymer may be a copolymer selected from the group consisting of acrylonitrile-maleic anhydride copolymer, acrylonitrile-vinylalcohol copolymer, cyanoethylene-vinylalcohol copolymer, cyanoethylene-cellulose copolymer, cyanoethylene-sucrose copolymer, acrylonitrile-acrylic acid copolymer, acrylonitrile-anhydrous maleic acid copolymer, acrylate-acrylic acid copolymer and acrylate-anhydrous maleic acid copolymer, or their mixtures. The organic/inorganic composite separator according to the present invention may be used for an electrochemical device such as a lithium secondary battery or a super capacity device as being interposed between a cathode and an anode. 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 sectional view schematically showing an organic/inorganic composite separator; FIG. 2 a photograph showing a surface of a porous coating layer and a surface of a polyethylene porous film of an organic/inorganic composite separator according to one embodiment of the present invention, taken by a SEM (Scanning Electron Microscope); FIG. 3 is a photograph showing an organic/inorganic composite separator according to one embodiment of the present invention, taken after the separator is left alone in an oven at 150℃ for 1 hour; FIG. 4 is a photograph showing a test device used for measuring a peeling force of a porous coating layer of the organic/inorganic composite separator; and 4 FIG. 5 is a graph showing measurement results of peeling forces of porous coating layers according to embodiments of the present invention and comparative examples. BEST MODE 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. An organic/inorganic composite separator of the present invention includes a porous substrate having a plurality of pores; and a porous coating layer formed with a plurality of inorganic particles and a binder polymer. Here, the binder polymer is a copolymer including (a) a first monomer unit having a contact angle to a water drop in the range from 0° to 49°; and (b) a second monomer unit having a contact angle to a water drop in the range from 50° to 130°. This copolymer may be expressed as follows: (first monomer unit)m-(second monomer unit)n (0