官方文档见
http://developer.apple.com/library/ios/#featuredarticles/XcodeConcepts/Concept-Targets.html#//apple_ref/doc/uid/TP40009328-CH4-SW1
官方文档见
http://developer.apple.com/library/ios/#featuredarticles/XcodeConcepts/Concept-Targets.html#//apple_ref/doc/uid/TP40009328-CH4-SW1