The visit and research of the first author was supported by “Abel visiting Scholar Program” of Commission for Developing Countries (IMU). The research was also supported by NSF of China (Nos. 11201130 and 11571216), Hunan Provincial Natural Science Foundation of China (No. 14JJ1012), and the construct program of the key discipline in Hunan province. The second author is currently on leave from the Indian Institute of Technology Madras.