supported by the Key Project of Chinese Ministry of Education(No.212135);Guangxi Natural Science Foundation(Nos.2012GXNSFB A053165);the Project of Education Department of Guangxi Autonomous Region(Nos.201203YB131,201202ZD071);Doctoral Initiating Project of Guangxi University of Technology(No.11Z09);the Fundamental Research Funds for the Central Universities(No.20112M0126)