Manuscript received November 27, 2008; revised February 23, 2009 and March 12, 2009. First published April 28, 2009; current version published July 1, 2009. This work was supported in part by the National Natural Science Foundation of China (60774043), in part by the National High Technology Research and Development of China (2008AA05Z211), and in part by the National Basic Research Program of China (973 Program) (2009CB219706).