The authors would like to present our sincere thanks to the referee for their valuable and helpful comments and suggestions. Also, the first author would like to thank Professor Dr. Zhen-hai Liu, who is my Ph.D. supervisor, for his valuable attention to our paper and would like to express my gratitude to Dr. Yun-xiang Li, Dr. Xiu-jin Miao for their useful discussions concerning this paper. This work was supported by the Graduate Degree Thesis Innovation Foundation of Central South University Grant No. CX2010B115 (PR China) the Doctoral Dissertation Innovation Project of Central South University Grant No. 2010ybfz016 (PR China) and NSF of Guangxi Grant No. 2010 GXNSFA 013114 (PR China).
