基金This work is supported in part by the National Natural Science Foundation of China under Grant Nos. 61332006, U1401256, 61328202, 61173029, the Fundamental Research Funds for the Central Universities of China under Grant No. N130504006, the Hong Kong RGC Project under Grant No. N_HKUST637/13, the National Basic Research 973 Program of China under Grant No. 2014CB340300, Microsoft Research Asia Gift Grant and Google Faculty Award 2013.