setTimeout的异步
http://www.cnblogs.com/littledu/articles/2607211.html
http://www.cnblogs.com/rubylouvre/archive/2009/08/20/1550264.html
http://www.cnblogs.com/littledu/articles/2607211.html
http://www.cnblogs.com/rubylouvre/archive/2009/08/20/1550264.html