Project supported by NNSF of China grant nos. 11271087, 61263006, NSF of the Department of Education of Hunan Province grant no. 13A013 and NSF of Guangxi grant no. 2014GXNSFDA118002, Special Funds of Guangxi Distinguished Experts Construction Engineering and Science Research Project 2014 of the China-ASEAN Study Center (Guangxi Science Experiment Center) of Guangxi University for Nationalities .