The project is supported by Natural Science Foundation of China, No. 11275093, the Science and Technology Key Project Foundation of Hunan Province, No. 2012GK2026 and Doctoral research foundation of the University of South China, No. 0201-2015XQD03. Cheng Wang provided the language help, and Professor Andy Lowenstein offered critical comments as well as correction of the grammar. Their enthusiastic help is gratefully acknowledged by the authors.