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 Pro-gram of Higher Education of China (Grant Nos. 20123515120016 and 20133515110014), the Natural Science Foundation of Fujian Province of China (Grant Nos. 2011J01079 and 2012J05040)