Acknowledgments 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. 10871079 and No. 11171370. The work of S. Ling was partially supported by Singapore MOE-AcRF Tier 2 Research Grant T208B2204. It is also the authors’ pleasure to thank the anonymous referees for their helpful comments.
