Foundation item: Projects(41171326, 40771198) supported by the National Natural Science Foundation of China; Project(08JJ6023) supported by the Natural Science Foundation of Hunan Province, China Received date: 2012−05−18; Accepted date: 2012−09−11 Corresponding author: ZENG Yong-nian, Professor, PhD; Tel: +86−13787795922; E-mail: ynzeng@csu-edu-cn-s.stat.lib.xjtu.edu.cn:443