广西自然科学基金项目No.2010GXNSFE013006,2011GXNSFA018123;广西科学研究与技术开发计划项目桂科攻No.0992027-1;广西教育厅科研项目No.201106LX546;Supported by the Natural Science Foundation of Guangxi No.2010GXNSFE013006,2011GXNSFA018123,the Guangxi Science and Technology Development ProgramNo.0992027-1 and the Scientific Research Fund of Guangxi Education DepartmentNo.201106LX546