Quite a part of this work was done while the first and third authors were visiting the Division of Mathematical Sciences, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore, in Autumn 2011. They are grateful for the hospitality and support. They also thank NSFC for the support through Grants No. 11271005 and No. 11171370 . Part of the work was also done when the second author was visiting the School of Mathematics and Statistics, Central China Normal University, Wuhan, in Spring 2012. The author acknowledges the support and hospitality received. The work of this author was partially supported by the National Research Foundation-Prime Minister's office, Republic of Singapore Competitive Research Programme NRF-CRP2-2007-03 . It is also the authors' pleasure to thank the anonymous referees for their helpful comments and suggestions.
