Foundation item: Projects(51104024, 51374043) supported by National Natural Science Foundation of China; Project(10JJ6019) supported by Hunan Provincial Natural Science Foundation, China; Project(10C0399) supported by Scientific Research Fund of Hunan Provincial Education Department, China; Project(2014SK3182) supported by Hunan Provincial Science & Technology Department, China Received date: 2013−10−14; Accepted date: 2014−01−16 Corresponding author: ZHONG Hong, Professor, PhD; Tel: +86−731−88830654; E-mail: zhongh@csu-edu-cn-s.stat.lib.xjtu.edu.cn:443