The work is funded by the Natural Science Fund of Hubei Province, China (No. 2008CDB046 ), the Natural Science Fund of China (Nos: 40771088 , 41001125 and 41071069 ), and the Science Fund of Chinese Universities for Central China Normal University (No. CCNU09Y01010 ). The authors thank Ananda Cohen and Yan Yu for editorial assistance and express our thanks to the anonymous reviewers and the Editor for useful suggestions for improvement. Any errors and views expressed in this paper are the sole responsibility of the authors.