The present investigation was supported by the National Natural Science Foundation under Grants 11101053 , 70971013 and 71171024 , the Natural Science Foundation of Hunan Province under Grant 09JJ1010 , the Key Project of Chinese Ministry of Education under Grant 211118 , the Excellent Youth Foundation of Educational Committee of Hunan Province under Grant 10B002 , the Open Fund Project of Key Research Institute of Philosophies and Social Sciences in Hunan Universities under Grant 11FEFM02 , and the Key Project of Natural Science Foundation of Educational Committee of Henan Province under Grant 12A110002 of the People’s Republic of China. The authors would like to thank the referee for his valuable comments and suggestions which essentially improved the quality of this paper.
