The authors would like to acknowledge the support from the International Science and Technology Cooperation Program of China (Grant No. 2014DFA11640 and 0903), the National Natural Science Foundation of China (NSFC) (Grant No. 61271224, 61301128 and 61471180), NFSC Major International Joint Research Project (Grant No. 61210002), the Hubei Provincial Science and Technology Department (Grant No. 2013BHE005), the Fundamental Research Funds for the Central Universities (Grant No. 2013ZZGH009 and 2014QN155), and EU FP7-PEOPLE-IRSES (Contract/Grant No. 247083, 318992 and 610524).