Supported by the National Natural Science Foundation of China(11201004 and 11271020);the Key Project of Chinese Ministry of Education(211077);the Anhui Provincial Natural Science Foundation(10040606Q30 and 1208085MA11)
supported by a grant from National Natural Science Foundation of China(10901003);NSF of Anhui Educational Bureau(KJ2009A128);Anhui Normal University Young Scientific Foundation(2008xqn44)