The research and the visit of the first author to ISI Chennai Centre were supported by “Abel visiting Scholar Program” of Commission for Developing Countries (IMU). The research was also supported by NSF of China (No. 11201130 ), Hunan Provincial Natural Science Foundation of China (No. 14JJ1012 ) and construct program of the key discipline in Hunan province. The second author is currently on leave from the Department of Mathematics, Indian Institute of Technology Madras, Chennai-600 036, India.