We thank two anonymous referees for their constructive suggestions and an editor (Brian Doonan) for help with English revision. This work was supported by the Introducing Advanced Technology Program (948 Program) (2010-4-03), the New Century Excellent Talents Program from the Ministry of Education, China (NCET-06-0715), the Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, and the Furong Scholar Program.