UIButton 的各种事件

参考这几个牛博就好：

http://feihu.me/blog/2015/trigger-condition-of-events-for-uibutton/

https://www.jianshu.com/p/c87428eb576c

http://www.cnblogs.com/niit-soft-518/p/4228701.html

https://www.jianshu.com/p/f7b716b5bb31