ios GCD 学习

http://blog.devtang.com/blog/2012/02/22/use-gcd/

http://developer.apple.com/library/ios/#documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html#//apple_ref/doc/uid/TP40008079-Reference-DontLinkElementID_1

http://www.2cto.com/kf/201209/157722.html