FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENT RULES, 2003 COMPLETE SPECIFICATION (See Section 10 and Rule 13) TITLE OF INVENTION: SEPARATOR HAVING POROUS COATING LAYER 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. TECHNICAL FIELD The present invention relates to a separator for an electrochemical device and an electrochemical device having the same. More particularly, the present invention relates to a separator in which a porous coating layer comprising a mixture of inorganic particles having different sizes and a binder polymer is formed on at least one surface of a porous substrate, and an electrochemical device having 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, 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 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 °C 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, attempts have been made to use a heat-resistant non-woven fabric made of a fiber having a melting or decomposition point higher than that of polyolefin without excessive thermal contraction as a separator. Meanwhile, a separator comprising a highly porous substrate and a porous organic/inorganic coating layer formed on at least one surface of the porous substrate wherein the porous coating layer is formed by coating with a mixture of an excess of inorganic particles and a binder polymer has been proposed. The porous organic/inorganic coating layer contains inorganic particles having superior heat resistance, which prevents short circuits between a cathode and an anode even when an electrochemical device is overheated. Such a porous organic/inorganic coating layer may be formed by pulverizing inorganic particles by means of a high-energy milling process, mixing the pulverized inorganic particles with a binder polymer to obtain a slurry, and coating the slurry obtained on a porous substrate. However, the high-energy milling process for pulverizing inorganic particles may cause a mechanochemical reaction in which the pulverized inorganic particles react with air or other raw materials to produce unnecessary substances. The mechanochemical reaction should be controlled. In particular, in the case functional particles are introduced in the porous organic/inorganic coating layer to control such a side reaction in an electrochemical device, this may cause an additional mechanochemical reaction by the new raw materials. DISCLOSURE Technical Problem The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a separator comprising a porous coating layer capable of minimizing a mechanochemical reaction to inhibit the production of unnecessary substances. It is another object of the present invention to provide a method for manufacturing the separator. It is still another object of the present invention to provide an electrochemical device comprising the separator. Technical Solution In accordance with one aspect of the present invention, there is provided a separator comprising a porous substrate; and a porous coating layer formed on at least one surface of the porous substrate and comprising a mixture of first inorganic particles coated with a coupling agent on the surface thereof, second inorganic particles coated with a coupling agent on the surface thereof and a binder polymer, the first inorganic particles having an average diameter of 1 to 10 μm and the second inorganic particles having an average diameter of 50 to 500 nm. The weight Tatio of the first inorganic particles : the second first inorganic particles preferably ranges from 50 : 50 to 90 : 10. The porous coating layer may further comprise functional particles. Examples of the functional particles may include, but are not limited to, a water scavenger, a Mn scavenger and an HF scavenger. The porous substrate used in the present invention may be a membrane or non-woven fabric form. The porous substrate may be made of a polyolefin-based polymer, but is not limited thereto. The polyolefin-based polymer is preferably selected from polyethylene, polypropylene, polybutylene and polypentene. Also, the first and the second inorganic particles used in the present invention may be selected from inorganic particles having a dielectric constant of 5 or higher, inorganic particles having the ability to transport lithium ions, and a mixture thereof. Examples of the inorganic particles having a dielectric constant of 5 or higher include BaTiO3, Pb(Zrx,Ti1-x)O3 (PZT, 0