来自 Ruanyifeng ,简单易懂.
http://www.ruanyifeng.com/blog/2011/08/a_detailed_explanation_of_jquery_deferred_object.html?20130219111528#comment-last
两位一篇是介绍 Deferred 实现.
http://www.cnblogs.com/nothingbrother/archive/2011/11/30/2269702.html
来自官方文档:
http://api.jquery.com/category/deferred-object/