iOS7 new for developer
https://developer.apple.com/library/ios/releasenotes/General/WhatsNewIniOS/Articles/iOS7.html
iOS7 UI transition guide
(en) https://developer.apple.com/library/ios/documentation/userexperience/conceptual/transitionguide/
(cn) http://mued.sohu.com/2013/06/ios-7-ui-transition-guide/
Tutorial: How to Update Your App for iOS 7
http://www.raywenderlich.com/49316/how-to-update-your-app-for-ios-7
Tutorial: What’s New in Objective-C and Foundation in iOS 7
http://www.raywenderlich.com/49850/whats-new-in-objective-c-and-foundation-in-ios-7