Import <Availability.h> in XCDYouTubeKit.h header

Remove `$(BUILT_PRODUCTS_DIR)/include` header search path. Let’s hope this will fix the "Build Succeeded" then "n Issues" right afterwards.
Update podspec
Update README for OS X
Update ARC requirements

It should not matter that ARC is required now.
Add PayPal donate button in README
Background playback for iOS 5, 6 and 7

Fixes #10
Move logging code out of the app delegate
Document that the completion handler is called on the main thread
CHANGELOG: Detailed demo projects improvements