We would like to thank the referees for their careful reading of the original manuscript and their valuable comments and suggestions that greatly improved the presentation of this paper. The work is supported by the National Natural Science Foundation of China (Nos. 11261017, 11371161) and the State Foundation for Studying Abroad. The authors thank Professor Binxiang Dai for his many valuable ideas and discussions. Also, the authors are grateful to Professor Yiping Chen and Doctor Changcheng Xiang for their help in numerical simulations.