关于javascript 回调函数
http://segmentfault.com/q/1010000000212522
如何避免Javascript中回调函数的嵌套?
http://javascript.ruanyifeng.com/jquery/deferred.html
http://www.ruanyifeng.com/blog/2011/08/a_detailed_explanation_of_jquery_deferred_object.html
出处:http://cnblogs.com/daishuguang