GEL-TYPED POLYMER ELECTROLYTE CONTAINING DIACRYL AMIDE-BASED POLYMERIC MATERIAL AND ELECTROCHEMICAL DEVICE COMPRISING THE SAME FIELD OF THE INVENTION The present invention relates to a gel polymer electrolyte containing a diacrylamide-based polymeric material and a secondary battery comprising the same. More specifically, the present invention relates to a secondary battery which is capable of achieving a significant reduction of thickness swelling by incorporation of a certain diacrylamide-based polymeric material into an electrolyte solvent and is also capable of achieving improved safety of the battery by prevention of electrolyte leakage from the battery. BACKGROUND OF THE INVENTION Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for batteries as an energy source. In order to cope with such a trend, a great deal of research and study has been focused on batteries which are capable of meeting various demands. Among other things, there has been an increasing demand for lithium secondary batteries such as lithium-ion batteries, lithium-ion polymer batteries and the like, which have high-energy density, high-discharge voltage and superior power output stability. Generally, lithium secondary batteries may be classified into lithium-ion batteries containing liquid electrolytes per se, lithium-ion polymer batteries containing liquid electrolytes in the form of gels, and lithium polymer batteries containing solid electrolytes, depending upon types of electrolytes to be employed. Particularly, the lithium-ion polymer batteries (or gel polymer batteries) have various advantages such as high safety due to lower probability of fluid leakage as compared to liquid electrolyte batteries, and feasible ultra-thinning and compactness of the battery shape and substantial weight reduction of the battery, which thereby lead to increased demands thereof. As representative methods for fabrication of the lithium-ion polymer battery, there are largely a fabrication method of a non-crosslinked polymer battery and a fabrication method of a directly-crosslinked polymer battery, depending upon kinds of matrix material for electrolyte impregnation. As the polymer matrix material, acrylate-and methacrylate-based materials having excellent radical polymerization reactivity, and ether-based materials having superior electrical conductivity are largely used. In particular, the latter directly-crosslinked polymer battery fabrication method is a method of fabricating a battery by placing a Jelly-roll type or stack type electrode assembly composed of electrode plates and a porous separator in a pouch, injecting a thermally polymerizable polyethylene oxide (PEO)-based monomer or oligomer crosslinking agent and an electrolyte composition thereto, and thermally curing the injected materials. The thus-fabricated battery has advantages of manufacturing processes in that plates and separators of conventional lithium-ion batteries can be directly employed without particular modifications or alterations. However, this method is known to suffer from disadvantages in that when the crosslinking agent is not completely cured and remained in the electrolyte, it is difficult to achieve uniform impregnation due to an increased viscosity, thereby significantly decreasing characteristics of the battery. Further, secondary batteries containing such a gel polymer electrolyte suffer from problems associated with deterioration of the battery safety due to leakage of the electrolyte which results from the occurrence of localized swelling of the battery thickness due to the precipitation of lithium metals from an anode during repeated charge/discharge cycles of the battery, since uniform distribution of the electrolyte into the electrode assembly is not achieved (see FIG. 1). Therefore, there is a strong need in the art for the development of a technology which is capable of securing the battery safety by preventing the thickness swelling while maintaining the battery performance. SUMMARY OF THE INVENTION Therefore, it is an object of the present invention to solve the above problems, and other technical problems that have yet to be resolved. As a result of a variety of extensive and intensive studies and experiments to solve the problems as described above, the inventors of the present invention have surprisingly discovered that, upon the preparation of a gel polymer electrolyte via thermal polymerization using diacrylamide monomers and/or oligomers, it is possible to secure the battery safety by significant suppression of thickness swelling of the battery to thereby prevent the electrolyte leakage, while having capacity performance comparable to that of conventional batteries utilizing liquid electrolytes. The present invention has been completed based on these findings. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. 1 is a view depicting an increase of a battery cell thickness in a conventional prismatic battery cell with respect to repeated cycles; and FIG. 2 is a graph showing changes in charge capacity and battery cell thickness with respect to increasing cycles, in test of Experimental Example 1 using batteries fabricated in Examples and Comparative Examples. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In accordance with an aspect of the present invention, the above and other objects can be accomplished by the provision of a gel polymer electrolyte comprising a diacrylamide compound as a precursor for formation of a crosslinked polymer. That is, the gel polymer electrolyte of the present invention comprises the electrolyte in the form of gel, using, as a matrix material, a crosslinked polymer formed by crosslinking of the diacrylamide compound as the precursor. Therefore, it is possible to prevent the electrolyte leakage by inhibiting the thickness swelling of the battery resulting from the precipitation of lithium metals from an anode during repeated charge/discharge cycles of the battery, due to non-uniform distribution of the electrolyte into the electrode assembly, consequently improving the safety of the battery containing such an electrolyte. More specifically, the diacrylamide compound has acrylamide groups and therefore exhibits high reactivity with radicals. Therefore, it is believed that the diacrylamide compound improves electrochemical stability of the final gel polymer electrolyte via an improved extent of reaction. Consequently, because a contact area of the electrolyte in contact with electrodes is decreased upon repeating charge/discharge of the battery, the thickness swelling of the battery is suppressed by inhibition of side reactions between the electrodes and the electrolyte arising from the decreased contact area of the electrolyte in contact with the electrodes, and by the reduced vapor pressure due to a gel polymer form of the electrolyte. Further, it is possible to minimize degradation of the battery performance, since there is no dissociation and decreased migration of lithium ions, which may occur in conventional gel polymer electrolytes, due to the presence of the polar functional groups, i.e. acrylamide groups. The crosslinked polymer utilized in the present invention refers to crosslinked products formed by polymerization of a diacrylamide monomer, an oligomer thereof, or the monomer and oligomer. That is, the crosslinked polymer of the present invention may be formed by crosslinking polymerization of the monomers or oligomers alone, or otherwise may be formed by simultaneous crosslinking polymerization of both the monomer and oligomer. As used herein, the term "oligomer" refers to a low-polymerization degree, linear polymer consisting of more than two monomers and having a viscosity to an extent that can be injected in the form of a solution. However, single use of the oligomer compound may lead to a difficulty to control physical properties, whereas single use of the monomer may result in a difficulty to obtain desired levels of mechanical properties. Therefore, a mixture of the high-molecular weight oligomeric compound and the monomeric compound may be preferably used to overcome such problems. In this case, a mixing ratio of the monomer and the oligomer may be in the range of 10:90 to 90:10 (w/w). Preferred examples of the diacrylamide compounds may include, but are not limited to, monomers represented by Formula I below and oligomers thereof: (Formula Removed) wherein R1 and R2 are each independently hydrogen or an unsubstituted or substituted C1-C6alkyl, and R1 and R2 may be taken together to form a saturated or unsaturated ring; and n is an integer of 0 to 4, and a direct bond is formed if n is 0. Particularly preferred examples of the diacrylamide compound may include monomers represented by Formulae II and III below and oligomers thereof: (Formula Removed) The above-mentioned compounds according to the present invention may be easily prepared by those skilled in the art, based on the chemical structure thereof, and therefore the details of preparation thereof are not provided herein. In one preferred embodiment, the gel polymer electrolyte may further comprise a compound that is polymerizable with the diacrylamide compound as a precursor for formation of a crosslinked polymer. There is no particular limit to the above-polymerizable compound, as long as such a compound is polymerizable with the diacrylamide compound. Preferably, mention may be made of (meth)acrylic ester compounds, unsaturated carboxylic compounds, vinyl compounds and mixtures thereof without being limited thereto. There is no particular limit to the (meth)acrylic ester compounds, as long as they contain acrylate group(s). Preferred are compounds containing two or more acrylate groups in the molecular structure. In one preferred embodiment, the (meth)acrylic ester compounds having two or more acrylate groups in the molecular structure may be diacrylate compounds. According to the results of the experiments conducted by the present inventors, it was confirmed that more flexible physical properties can be obtained upon preparation of the gel polymer electrolyte, by the combined use of a diacrylate compound in conjunction with the diacrylamide compound. That is, the gel polymer electrolyte is obtained which has combination of electrochemical properties and mechanical properties of each material, by the co-use of the diacrylamide compound having superior binding force with the diacrylate compound having superior elasticity, as a precursor of a crosslinked polymer. Of course, the diacrylate compound, in conjunction with the diacrylamide compound, may form various forms of copolymers, for example random copolymers, block copolymers, graft copolymers and the like. Preferred examples of the diacrylate compound may include, but are not limited to, monomers represented by Formula IV below, and oligomers thereof:(Formula Removed) wherein R3, R4 and R5 are each independently hydrogen, or an unsubstituted or substituted C1-C4alkyl; and m is an integer of 1 to 20. Examples of the (meth)acrylic ester compounds having two or more acrylate groups in the molecular structure may include, but are not limited to, diethylene glycol diacrylate (Di(EG)DA), diethylene glycol dimethacrylate (Di(EG)DM), ethylene glycol dimethacrylate (EGDM), dipropylene glycol diacrylate (Di(PG)DA), dipropylene glycol dimethacrylate (Di(PG)DM), ethylene glycol divinyl ether (EGDVE), ethoxylated (6) trimethylolpropane triacrylate (ETMPTA), diethylene glycol divinyl ether (Di(EG)DVE), triethylene glycol dimethacrylate (Tri(EG)DM), dipentaerythritol pentaacrylate (DPentA), trimethylolpropane triacrylate (TMPTA), trimethylolpropane trimethacrylate (TMPTM), propoxylated (3) trimethylolpropane triacrylate (PO(3)TMPTA), propoxylated (6) trimethylolpropane triacrylate (PO(6)TMPTA), poly(ethylene glycol)diacrylate (PA1, Mn = 700, see Formula III), poly(ethylene glycol) dimethacrylate and mixtures thereof. Preferably, the gel polymer electrolyte contains a polymerization initiator, an electrolyte and a lithium salt. Examples of the polymerization initiator may include azo compounds such as 2,2-azobis(2-cyanobutane), 2,2-azobis(methylbutyronitrile), 2,2 ' -azoisobutyronitrile (AIBN), azobisdimethyl-valeronitrile (AMVN) and the like, peroxy compounds such as benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide, cumyl peroxide, hydrogen peroxide and the like, and hydroperoxides. Preferably, AIBN, 2,2'-azobis(2,4-dimethyl valeronitrile) (V65), Di-(4-tert-butylcyclohexyl)-peroxydicarbonate (DBC), or the like may be employed. The polymerization initiator is decomposed at a certain temperature of 40 to 80 °C to form radicals, and may react with monomers via the free radical polymerization to form a gel polymer electrolyte. Generally, the free radical polymerization is carried out by sequential reactions consisting of the initiation involving formation of transient molecules having high reactivity or active sites, the propagation involving re-formation of active sites at the ends of chains by addition of monomers to active chain ends, the chain transfer involving transfer of the active sites to other molecules, and the termination involving destruction of active chain centers. On the other hand, it is, of course, possible to carry out polymerization without use of the polymerization initiator. The electrolyte may also serve as a plasticizer. As examples of the electrolyte that can be used in the present invention, mention may be made of non-protic organic solvents such as N-methyl-2-pyrollidinone, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl carbonate (EMC), gamma-butyrolactone, 1,2-dimethoxy ethane, tetrahydroxy Franc, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane, dioxolane derivatives, sulfolane, methyl sulfolane, l,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate and ethyl propionate. These materials may be used alone or in any combination thereof. The lithium salt is a material that is dissolved in the non-aqueous electrolyte to thereby resulting in dissociation of lithium ions. Examples of the lithium salt may include, for example, LiCl, LiBr, Lil, LiClO4, LiBF4, LiB10Cl10, LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, CF3SO3Li, (CF3SO2)2NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide. These materials may be used alone or in any combination thereof. Additionally, in order to improve charge/discharge characteristics and flame retardancy, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxy ethanol, aluminum trichloride or the like may be added to the electrolyte. If necessary, in order to impart incombustibility, the electrolyte may further include halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride. Further, in order to improve high-temperature storage characteristics, the electrolyte may additionally include carbon dioxide gas. Since the gel polymer electrolyte according to the present invention is included in the gel-type, instead of a liquid phase, the ratio of the electrolyte : the crosslinked polymer formed by crosslinking of the diacrylamide compound is important in order to achieve uniform application of the electrolyte to the electrodes. Although there is no particular limit to a content of the diacrylamide compound, it is preferred to contain the diacrylamide compound in an amount of 0.1 to 10% by weight based on the total weight of the electrolyte. Where the ratio of the crosslinked polymer is lower than 0.1% by weight, it is not easy to form a gel polymer, consequently resulting in significant swelling of the battery which occurs upon use of the liquid electrolyte, and it may also be difficult to prepare a matrix material having a given thickness. On the other hand, where the content of the crosslinked polymer exceeds 10% by weight, an increased density of the gel polymer may lead to a decreased transfer rate of lithium ions, which in turn causes the precipitation of lithium ions, consequently resulting in the deterioration of the battery performance, and may also lead to an increased viscosity, thereby presenting a difficulty to achieve uniform application thereof to target sites. Addition of the diacrylate compound to the diacrylamide compound also suffers from the same problems as described above. That is, the total weight of the diacrylamide compound and the diacrylate compound is preferably 1 to 10% by weight, based on the total weight of the electrolyte. In accordance with another aspect of the present invention, there is provided an electrochemical device comprising the above-mentioned gel polymer electrolyte. The electrochemical device encompasses all kinds of devices that undergo electrochemical reactions. As specific examples of the electrochemical device, mention may be made of all kinds of primary batteries, secondary batteries, fuel cells, solar cells, capacitors and the like. Preferred are secondary batteries. Generally, the secondary battery is fabricated by inclusion of the electrolyte in an electrode assembly composed of a cathode and an anode, which are faced opposite to each other with a separator therebetween. The cathode is, for example, fabricated by applying a mixture of a cathode active material, a conductive material and a binder to a cathode current collector, followed by drying and pressing. If necessary, a filler may be further added to the above mixture. The cathode current collector is generally fabricated to have a thickness of 3 to 500 µm. There is no particular limit to materials for the cathode current collector, so long as they have high conductivity without causing chemical changes in the fabricated battery As examples of the materials for the cathode current collector, mention may be made of stainless steel, aluminum, nickel, titanium, sintered carbon, and aluminum or stainless steel which was surface-treated with carbon, nickel, titanium or silver. The current collector may be fabricated to have fine irregularities on the surface thereof so as to enhance adhesion to the cathode active material. In addition, the current collector may take various forms including films, sheets, foils, nets, porous structures, foams and non-woven fabrics. Examples of the cathode active materials that can be used in the present invention may include, but are not limited to, layered compounds such as lithium cobalt oxide (LiCo()2) and lithium nickel oxide (LiNiCh), or compounds substituted with one or more transition metals; lithium manganese oxides such as compounds of Formula Li]+xMn2-xO4 (0