http://www.devdiv.com/forum.php?mod=viewthread&tid=128378&page=1#pid743150
http://www.devdiv.com/iOS_iPhone-ios_uicollectionview_circlelayout-thread-128893-1-1.html
http://onevcat.com/2012/06/introducing-collection-views/
http://www.objccn.io/issue-3-3/
http://objccn.io/issue-5-2/
http://kittenyang.com/layout-algorithm/
http://www.jianshu.com/p/b1a07f65c2c3
http://www.jianshu.com/p/3ca4158e1e6e
http://www.jianshu.com/p/4291bcade051
http://www.jianshu.com/p/df431a702135
