The authors would like to thank the editor and anonymous reviewers for their helpful comments and valuable suggestions, which have greatly improved the quality of this paper. This work was financially supported by the National Natural Science Foundation of China (Grant No. 11361012), the China Postdoctoral Science Foundation(No.2013T60934), the Scientific Research Foundation of Guizhou Provincial Science and Technology Department([2013]J2083), and "125" Science and Technology Grand Project Sponsored by the Department of Education of Guizhou Province ([2012]011).