https://github.com/search?l=Objective-C&p=2&q=cocos&ref=searchbar&type=Repositories
http://www.csdn.net/article/2010-09-09/279255
http://stackoverflow.com/questions/10614876/rotate-sprite-to-face-a-point-cocos2d
http://stackoverflow.com/questions/7785089/cocos2d-angle-confusion
http://stackoverflow.com/questions/6064630/get-angle-from-2-positions
http://www.nipic.com/show/4/79/4807200k6e741f91.html
http://www.nipic.com/show/4/79/4311521kd4be8f0a.html