Acknowledgements
This work was supported by the National Natural Science Foundation of China (Grant Nos. 30671464, 30972070, 311711776 and 31201445), the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant Nos. 20123515120016 and 20133515110014), the Natural Science Foundation of Fujian Province of China (Grant Nos. 2011J01079 and 2012J05040)