This work was supported by National Natural Science Foundation of China (Grant Nos. 11201177 and 11271147), China Postdoctoral Science Foundation (Grant No. 2013M531716), Scientific Research Plan Item of Hunan Provincial Department of Education (Grant No. 12C0287), Jishou University Doctor Science Foundation (Grant No. jsdxxcfxbskyxm201113), and Scientific Research Plan Item of Jishou University (Grant No. 13JDY041). The authors greatly appreciate the helpful suggestions from the referees, editor and associate editor-in-chief.