We gratefully acknowledge financial support from the NSFC
(Grant Nos. 21072083 and 21272104), 973 Program (Grant
2010CB833203), the program for Changjiang Scholars and
Innovative Research Team in University (PCSIRT: IRT1138),
the fund for the Doctoral Program of Higher Education of
China (No. 20110211110009), and the Fundamental Research
Funds for the Central Universities (Nos. lzujbky-2012-56,
lzujbky-2013-ct02).