参照:http://blog.csdn.net/growinggiant/article/details/41077221
http://codingobjc.com/blog/2013/05/07/gcdshi-yong-xiang-jie-xia-pian/
http://tutuge.me/2015/04/03/something-about-gcd/
http://justsee.iteye.com/blog/2233252
