FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENT RULES, 2003 COMPLETE SPECIFICATION (See Section 10 and Rule 13) TITLE OF INVENTION: A SEPARATOR, MANUFACTURING METHOD OF THE SAME, AND ELECTROCHEMICAL DEVICE HAVING THE SAME APPLICANT: LG CHEM, LTD. A company incorporated in Republic of Korea 128, Yeoui-daero Youngdungpo-gu Seoul 150-721, Republic of Korea The following specification particularly describes the invention and the manner in which it is to be performed. 2 TECHNICAL FIELD The present invention relates to a separator used in an electrochemical device such as a lithium secondary battery, a method for manufacturing the same and an electrochemical device having the same. More particularly, the present invention relates to a separator in which porous coating layers comprising a mixture of inorganic particles and a binder polymer are formed on the surface of a porous substrate, a method for manufacturing the same and an electrochemical device having the same. This application claims priority to Korean Patent Application No. 10-2011-0071799 filed in the Republic of Korea on July 20, 2011, the entire contents of which are incorporated herein by reference. This application claims priority to Korean Patent Application No. 10-2012-0079209 filed in the Republic of Korea on July 20, 2012, the entire contents of which are incorporated herein by reference. BACKGROUND ART Recently, there has been an increasing interest in energy storage technology. As energy storage technologies are extended to devices such as cellular phones, camcorders and notebook PC, and further to electric vehicles, demand for the research and development of electrochemical devices is increasing. 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, research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy. Many secondary batteries are currently available. Among these, lithium secondary batteries developed in the early 1990’s have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, for example, Ni-MH, Ni-Cd, and H2SO4-Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire and explosion, when encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. More research is still 3 urgently needed to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries. Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, regulatory guidelines strictly restrict potential dangers (such as fire and smoke emission) of electrochemical devices. Overheating of an electrochemical device may cause thermal runaway or a puncture of a separator may pose an increased risk of explosion. In particular, porous polyolefin substrates commonly used as separators for electrochemical devices undergo severe thermal shrinkage at a temperature of 100 ºC or higher in view of their material characteristics and production processes including elongation. This thermal shrinkage behavior may cause electrical short between a cathode and an anode. In order to solve the above safety problems of electrochemical devices, a separator comprising a highly porous substrate and a porous organic/inorganic composite coating layer formed on at least one surface of the porous substrate by coating with a mixture of inorganic particles and a binder polymer has been proposed. For example, Korean Laid-open Patent Publication No. 2007-0019958 discloses a separator comprising a porous substrate and a porous coating layer formed on the surface of the porous substrate by using a mixture of inorganic particles and a binder polymer. Such a porous coating layer formed on a porous substrate contributes to the improvement of safety in electrochemical devices. In the prior art, inorganic particles such as BaTiO3, Pb(Zr,Ti)O3 (PZT), ZrO2, SiO2, Al2O3, TiO2, lithium phosphate (Li3PO4) and lithium titanium phosphate (LixTiy(PO4)3, 0 < x < 2, 0 < y < 3) have been used to form a porous coating layer, however, fail to improve the output of electrochemical devices due to their electrochemical characteristics. As an attempt to improve the output of electrochemical devices, Korean Laid-open Patent Publication No. 2008-101043 discloses a separator having a porous coating layer comprising active material particles. However, the porous coating layer comprising active material particles has poor conductivity between active materials due to the use of a binder polymer and thus is preferable to further comprise a conductive material. The conductive material may have a particle size as small as several tens of nanometers, whereas a porous substrate has at least a pore size of several tens of micrometers. Therefore, the conductive material penetrates into 4 pores of the porous substrate to provide conductivity, thereby causing a short circuit between electrodes. DISCLOSURE Technical Problem Accordingly, it is an object of the present invention to provide a separator capable of improving the output of electrochemical devices with a porous layer comprising an active material together with a conductive material and preventing a short circuit due to the use of the conductive material, and a method for manufacturing the separator. Technical Solution In accordance with one aspect of the present invention, there is provided a separator which comprises a porous composite having a porous substrate and a first porous coating layer formed on at least one surface of the porous substrate and comprising a mixture of inorganic particles and a first binder polymer; and a second porous coating layer formed on a first surface of the porous composite and comprising a mixture of cathode active material particles, a second binder polymer and a first conductive material, a third porous coating layer formed on a second surface of the porous composite and comprising a mixture of anode active material particles, a third binder polymer and a second conductive material, or both of the second porous coating layer and the third porous coating layer. The separator may further comprise a fourth porous coating layer formed on at least one outermost surface thereof and comprising a fourth binder polymer. The cathode active material particles which may be used in the present invention include LiCoO2, LiNiO2, LiMn2O4, LiCoPO4, LiFePO4, LiNiMnCoO2 and LiNi1-x-y-zCoxM1yM2zO2 (wherein, M1 and M2 are each independently selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg and Mo, x, y and z are each independently an atomic fraction of elements consisting of an oxide, and 0 ≤ x < 0.5, 0 ≤ y < 0.5, 0 ≤ z < 0.5, and x+y+z<1). The anode active material particles which may be used in the present invention include natural graphite, artificial graphite, a carbon-based material, LiTi2O4, silicon (Si) and tin (Sn). Among these, the preferred cathode active material particles may be made of LiFePO4 and the preferred anode active material particles may be made of LiTi2O4. The conductive material which may be used in the present invention includes carbon 5 black, acetylene black, Ketjen Black, Super P and carbon natotubes. The inorganic particles which may be used in the present invention include inorganic particles having a dielectric constant of 5 or higher and inorganic particles having the ability to transport lithium ions, but are not particularly limited thereto. Examples of the inorganic particles having a dielectric constant of 5 or higher include BaTiO3, Pb(Zrx,Ti1-x)O3 (PZT, 0