Foundation item: Project(51178061) supported by the National Natural Science Foundation of China; Project(2010FJ6016) supported by Hunan Provincial Science and Technology, China; Project(12C0015) supported by Scientific Research Fund of Hunan Provincial Education Department, China; Project(13JJ3072) supported by Hunan Provincial Natural Science Foundation of China Received date: 2012−05−07; Accepted date: 2012−07−11 Corresponding author: LI Li-hua, PhD; Tel: +86−731−85258575; E-mail: hbxiaoli98@163-com-s.stat.lib.xjtu.edu.cn:443