GCD深入理解一
http://www.cocoachina.com/industry/20140428/8248.html
GCD深入理解二
http://www.cocoachina.com/industry/20140515/8433.html
NSthread NSOperation GCD
http://www.cocoachina.com/industry/20140520/8485.html
原文：http://www.cnblogs.com/fisland/p/4318182.html