Acknowledgements
This study was supported by SingaporeMinistry of Education
Academic Research Fund Tier 1 (R-143-000-583-112) and the Na-
tional University of Singapore start-up grant (R-143-000-561-133).
Projects 31371851, 31071617, 31471605 and 31200801 supported
by NSFC, and Natural Science Foundation of Jiangsu Province
(BK20141220) also contributed to this research.