The authors wish to thank Mr. Chao Yang for valuable advices. This work was supported by the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (No. 201321), the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130002120013), and the National Natural Science Foundation of China (Nos. 21521091, 21131004, 21390393, 21322107, 21325101, 21471089, and U1463202).
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