The authors are most grateful to the Editor prof. A Volodin and the anonymous referees for carefully reading the paper and for offering valuable suggestions, which greatly improved this paper. This paper is supported by the National Nature Science Foundation of China (11526085, 71501025, 11401127), the Humanities and Social Sciences Foundation for the Youth Scholars of Ministry of Education of China (15YJCZH066), the Guangxi Provincial Natural Science Foundation of China (2014GXNSFBA118006, 2014GXNSFCA118015), the Scientific Research Fund of Guangxi Provincial Education Department (2013YB104), the Construct Program of the Key Discipline in Hunan Province (No. [2011]76), and the Science Foundation of Hengyang Normal University (14B30).