The authors would like to thank Prof. Yuanhong Yu and the anonymous reviewer fortheir constructive and valuable comments, which have contributed much to theimproved presentation of this paper. This work was supported by the NationalScience and Technology Major Projects of China (Grant No. 2012ZX10001001-006),the National Natural Science Foundation of China (Grant No. 11101053, 7117102471371195), the Scientific Research Fund of Hunan Provincial Education Departmentof China (Grant No. 11A008) and the Planned Science and Technology Project ofHunan Province of China (Grant No. 2012SK3098, 2013SK3143).