基金This research received financial support from the National Natural Science Foundation of China （Grant Nos. 40972090, 40672143 and 40172076）, the National Major Fundamental Research and Development Project （Grant Nos. 2005CB422107 and G1999043305）, and the National Science and Technology Project （Grant Nos. 2008ZX05005-002-04-02）. The authors sincerely thank Ma Zongjin, Jia Chengzao, Jin Zhijun, Liu Hefu and others for helpful discussions and suggestions for improving the manuscript.