http://blog.csdn.net/mxiaoyem/article/details/50857008
http://blog.csdn.net/ke1vin/article/details/51130380
http://www.jianshu.com/p/f61849073bad
http://blog.csdn.net/wangbaochu/article/details/45729143
https://github.com/ldoublem/LoadingView