CATHODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY FIELD OF THE INVENTION The present invention relates to a cathode active material for a lithium secondary battery and, more particularly, to a cathode active material including a lithium-transition metal composite oxide which has a specific composition, includes an excess of lithium and, as a constitutional element, nickel having a predetermined oxidation number, so as to exhibit a stable crystal structure and excellent rate characteristics under high rate charge/discharge conditions. BACKGROUND OF THE INVENTION With increased development of technologies for mobile devices and requirements for the same, a demand for a secondary battery as an energy source of the mobile device has rapidly increased. In particular, a lithium secondary battery having high energy density and operating potential, a relatively long lifespan, and a low self-discharge rate is commercially available and widely used in the art. Further, increasing interest in environmental problems leads to a great deal of study and investigation into electric vehicles and/or hybrid vehicles which may be employed in place of commercial vehicles using fossil fuels such as gas engine vehicles, diesel engine vehicles, etc. Such a hybrid vehicle or electric vehicle mostly uses a nickel hydrogen metal secondary battery as a power source and, in recent years, a variety of strategies are actively under investigation to apply a lithium secondary battery with high energy density and discharge voltage to the hybrid electric vehicle and some of them are now commercially available. As a cathode active material for the lithium secondary battery, lithium containing cobalt oxides (L1COO2) are generally used. However, using other materials, for example, lithium containing manganese oxides such as LiMnO2 with a lamellar crystal structure, LiMn2O4 with a spinel structure, and the like and/or lithium containing nickel oxides (LiNiO) may also be taken into consideration. Among various cathode active materials, LiCoC^ with excellent lifespan and superior charge/discharge efficiency is generally used. However, this substance has a significant problem of insufficient price competitiveness since cobalt is expensive as a limited resource. Although lithium manganese oxides such as LiMnCh, LiMn2O4, etc. have merits of excellent thermal stability, low cost and easy synthesis thereof, these compounds entail some disadvantages of low capacity, deteriorated high-temperature characteristics and low conductivity. In addition, LiNiO2 based cathode active materials are relatively inexpensive and have a high discharge capacity. However, when exposed to atmosphere and/or moisture, these materials exhibit marked phase transfer in crystal structure depending on a variation in volume associated with a charge/discharge cycle. In order to overcome these difficulties, a great deal of research and investigation into use of a lithium oxide including nickel and manganese in a relative ratio of 1:1 or nickel, cobalt and manganese in a relative ratio of 1:1:1 as a cathode active material have been conducted. The cathode active material prepared by mixing nickel, cobalt and/or manganese exhibited more enhanced characteristics than a cathode active material including only any one of these transition metals. However, there are still requirements for simplifying production processes and for improving high rate characteristics of a battery. In order to solve the conventional problems described above, the present invention provides a cathode active material including a lithium-transition metal composite oxide, wherein each constitutional element of the composite oxide has predetermined composition and oxidation number. As for technical concepts in regard to the present invention, Korean Laid-Open Application No. 2005-047291 and PCT International Laid-Open Application No. WO 2002-078105 describe an oxide represented by a compositional formula of Lil+xNii/2Mni/202 (02+, Ni"=3+, Mn=4+ and Co=3+; 02+, Ni"=3+, Mn=4+ and Co=3+ and maintaining desired ratios of constitutional elements depending on excess lithium by the above equation, a cathode active material may have a stable crystal structure so that lithium ion mobility and rate characteristics may thereby be improved. That is, as for a lithium-transition metal composite oxide contained in the cathode active material of the present invention, an average oxidation number of Ni' may be at least 2 while an average oxidation number of Ni" may be at least 3, which are both higher than an average oxidation number of Ni commonly used in the conventional cathode active material. As a result, a stable bonding structure and a high bonding force between a transition metal element and oxygen may be attained. This result is expected because electric charge is increased as the oxidation number of Ni increases, resulting in an increase in coulombic force between the transition metal element and oxygen. Therefore, the cathode active material of the present invention may have a stable crystal structure and ensure a desirable path required for rapid movement of lithium ions under high rate charge/discharge conditions, thereby remarkably enhancing rate characteristics while maintaining stability in crystal structure. 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: FIGS. 1 to 5 are graphs illustrating rate characteristics of cathode active materials according to Examples 1 to 12, compared to those according to Comparative Examples 1 to 11; and FIGS. 6 to 10 are each pairs of graphs illustrating measured lattice constants (a and c) for cathode active materials according to Examples 1 to 4, Examples 5 and 6, Examples 7 and 8, Examples 9 and 10, and Examples 11 and 12, respectively. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS An excess of lithium contained in a cathode active material according to the present invention means an amount of lithium sufficient to prevent deterioration in structural stability of a lithium-transition metal composite oxide caused by the excess lithium and, at the same time, to provide enhanced rate characteristics thereof under high rate charge/discharge conditions. As defined by the above compositional formula, a value calculated by dividing a molar fraction of lithium (which is contained in the cathode active material) by the sum of molar fractions of all transition metals (which are contained in the cathode active material) may be at least 1.1 and less than 1.3, and preferably, may range from 1.1 to 1.2. Molar ratios of nickel and manganese may independently range from 0.2 to 0.55 and, a molar ratio of each element in the composite oxide described above may be flexibly adjusted relative to the excess lithium. In a preferable embodiment, a relative molar ratio of nickel to manganese may range from 1:0.7 to 1.3, meaning that an amount of manganese may be greater or less than that of nickel. However, an absolute value of a difference between molar ratios of nickel and manganese is always set to less than 0.1 so that excellent physical properties of nickel and manganese, respectively, may be balanced in the cathode active material. Compared to a conventional three-component based cathode active material including transition metals such as nickel, manganese and/or cobalt wherein nickel maintains an oxidation number of 2, an average oxidation number of nickel in the composite oxide included in the cathode active material according to the present invention may vary depending on an amount of excess lithium contained in the composite oxide. In other words, the composite oxide included in the cathode active material according to the present invention may contain Ni2+ and Ni3+ simultaneously. More particularly, as for the average oxidation number of nickel, if the molar ratio of nickel (that is, b+c) is greater than that of manganese (d), Ni' is more than 2+ and Ni" is 3+ and, in this case, the excess nickel above the molar ratio of nickel to the molar ratio of manganese may be Ni". This Ni" often replaces a metal element located at a site on which the excess lithium having the oxidation number of 1 is combined with oxygen, thus having an oxidation number of 3 in order to maintain the predetermined total oxidation number. Herein, a nickel moiety in an amount corresponding to the molar ratio of manganese may be Ni' and the average oxidation number of nickel may be at least 2 because of simultaneous existence of Ni2+ and Ni3+ and influence of the excess lithium. Moreover, even if the molar ratio of nickel (b+c) is smaller than that of manganese (d), the average oxidation number of Ni' may be at least 2 so as to maintain the predetermined total oxidation number, due to the influence of the excess lithium. A molar ratio of cobalt is in general not more than 0.1 and this element may be included in a minimum amount and, occasionally, may not be contained in consideration of economical aspects based on resources and material cost. So long as the molar ratios of nickel and manganese and the excess lithium in a cathode active material of the present invention are maintained as defined by formula (1), a process for preparation of the cathode active material is not particularly limited. For example, the cathode active material may be prepared by reaction of a nickel-manganese-(cobalt) hydroxide precursor with lithium carbonate. More particularly, preparing a nickel-manganese-(cobalt) hydroxide with a constitutional composition sufficient to prepare an oxide containing an excess of lithium after reaction, the prepared hydroxide is mixed with a lithium containing precursor to proceed reaction thereof and the reacted mixture is calcined at 800 to 1,200 "C for 8 to 24 hours to produce the cathode active material as a final product. The present invention also provides a lithium secondary battery including the cathode active material produced above. In general, a lithium secondary battery includes a cathode, an anode, a separation membrane and a non-aqueous electrolyte containing a lithium salt. The cathode may be formed by applying a mixture of a cathode active material, a conductive material and a binder to a cathode (current) collector, drying the coated collector and pressing the treated collector and, if necessary, the mixture may additionally include filler. The cathode collector may have a thickness ranging from 3 to 500jtan. Such cathode collector is not particularly restricted so long as it does not induce chemical modification of a battery while having excellent conductive properties. For example, the cathode collector may include stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., and so forth. The cathode collector may have microfine roughness on a surface thereof so as to reinforce adhesion of the cathode active material and, in addition, may be fabricated in various forms such as a film, a sheet, a foil, a net, a porous material, a foamed material, a non-woven fabric material, and the like. The conductive material may be added in an amount of 1 to 50wt.% of the total weight of a mixture containing the cathode active material. Such conductive material is not particularly restricted so long as it does not cause chemical modification of a battery while having desired conductivity. For instance, the conductive material may include: graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, etc.; conductive fiber such as carbon fiber, metal fiber, etc.; metal powder such as carbon fluoride, aluminum, nickel powder, etc.; conductive whiskers such as zinc oxide, potassium titanate, etc.; conductive metal oxides such as titanium oxide; polyphenylene derivatives, and so forth. The binder used herein may comprise a component for supporting combination of an active material with the conductive material and/or binding to the collector and, in general, may be added in an amount of 1 to 50wt.% of the total weight of a mixture comprising a cathode active material. Such binder may include, for example, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butyrene rubber, fluorine rubber, various copolymers, etc. The filler used herein may optionally be used to inhibit expansion of a cathode and, is not particularly restricted so long as it is a fibrous material which does not cause chemical modification of a battery. For example, the filler may include an olefin based polymer such as polyethylene, polypropylene, etc.; or a fibrous material such as glass fiber, carbon fiber, etc. An anode is fabricated by applying an anode active material to an anode collector and drying the coated collector and may optionally include other components described above. The anode collector generally has a thickness of 3 to 500fM- Such anode collector is not particularly restricted so long as it does not induce chemical modification of a battery while having favorable conductive properties. For example, the anode collector may include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, and so forth. Similar to a cathode collector, the anode collector may have microfine roughness on a surface thereof so as to reinforce adhesion of the anode active material, and the anode collector may be fabricated in various forms such as a film, a sheet, a foil, a net, a porous material, a foamed material, a non-woven fabric material, and the like. The anode active material may include, for example: carbon such as graphitization retardant carbon, graphite based carbon, etc.; metal composite oxides such as LiyFe2O3 (02+, Ni"=3+, Mn=4+ and Co=3+; O