The authors would like to thank the reviewers for their detailed comments that have helped to improve the quality of the paper, and thank Software engineering Postdoctoral Station of Central South University for supporting research environment. This work is supported by National Natural Science Foundation of China under Grants No. 61309027, 61272151, and Scientific Research Fund of Hunan Provincial Education Department No. 13B148, and supported by China Postdoctoral Science Foundation No.2013M542136, and supported by Hunan Province postdoctoral research Foundation No. 2014RS4027,and supported by the Postdoctoral foundation of Central South University No.126224, and supported by Youth Scientific Research Foundation of Central South University of Forestry and Technology No. QJ2011002A.