fix the issue5

https://github.com/xhzengAIB/MessageDisplayKit/issues/5
1、Add subClass due with weChat demo.
2、WeChat demo test add / remove / inserts message.
3、Change the XHMessageTableViewController add / remove / inserts method.
4、configure Avator
5、Highly customizable at MessageTableViewCell / BubbleMessageView.
6、Add define due with iPad or no.
7、Refactor  Message model
8、Add Message BackgrounImage png.
delete png file.