UICollectionViewLayout布局

蜂窝布局：
http://blog.csdn.net/duzixi/article/details/39010887
瀑布流布局：
http://www.cnblogs.com/YouXianMing/p/3977654.html
布局详解&方，圆，线形状布局
http://www.jianshu.com/p/45ff718090a8