The authors would like to thank the anonymous reviewers for their valuable comments and suggestions. This work was partially supported by the National Natural Science Foundation of China (Grant Nos. 61070057 ), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China (Grant No. 708066 ), the Hunan provincial Science and Technology important Projects (Grant No. 2010FJ1006 ), the Hunan Provincial Innovation Foundation For Postgraduate (Grant No. CX2010B139 ), and A Project Supported by Scientific Research Fund of Hunan Provincial Education Department ( 08C435 ).