We would like to thank Professor Ying Wu for his encouragement and helpful discussion. This research was supported in part by the National Natural Science Foundation of China under Grant nos. 11404250 , 11574104, 11505131, 11447107, U1504111, and 11204223 , by the Talent Scientific Research Foundation of Wuhan Polytechnic University under Grant no. 2014RZ20 , and by the National Basic Research Program of China under Contract no. 2012CB922103 .