基金This research was partially supported by National Natural Science Funds for Distinguished Young Scholar under Grant No. 70825004 and National Natural Science Foundation of China (NSFC) under Grant No. 10731010, the National Basic Research Program under Grant No. 2007CB814902, Creative Research Groups of China under Grant No.10721101 and Shanghai University of Finance and Economics through Project 211 Phase III and Shanghai Leading Academic Discipline Project under Grant No. B803.