Use the new nullability annotations for XCDYouTubeClient
Use the new nullability annotations for XCDYouTubeClient

In recent versions of clang (Xcode 6.3 or 6.4) the optimizer became more agressive and optimized out code testing the nullability of a parameter declared with __attribute__((nonnull)). This is why `testNilCompletionHandler` started to fail. The new nullability annotations are not handled by the optimizer.

ramoslin02/XCDYouTubeKit,mingsai/XCDYouTubeVideoPlayerViewController,mingsai/XCDYouTubeVideoPlayerViewController,itamaker/XCDYouTubeKit,morrisirrom/XCDYouTubeKit,twobitlabs/XCDYouTubeKit,gshaviv/XCDYouTubeVideoPlayerViewController,kumabook/XCDYouTubeKit,0xced/XCDYouTubeKit,creationst/XCDYouTubeKit,kumabook/XCDYouTubeKit,ramoslin02/XCDYouTubeKit,donpark/XCDYouTubeKit,mingsai/XCDYouTubeVideoPlayerViewController,yjkim/XCDYouTubeKit,twobitlabs/XCDYouTubeKit,kumabook/XCDYouTubeKit,ramoslin02/XCDYouTubeKit,donpark/XCDYouTubeKit,creationst/XCDYouTubeKit,gshaviv/XCDYouTubeVideoPlayerViewController,yjkim/XCDYouTubeKit,creationst/XCDYouTubeKit,0xced/XCDYouTubeKit,gshaviv/XCDYouTubeVideoPlayerViewController,yjkim/XCDYouTubeKit,yjkim/XCDYouTubeKit,0xced/XCDYouTubeKit,c0unt0/XCDYouTubeKit,gshaviv/XCDYouTubeVideoPlayerViewController,ramoslin02/XCDYouTubeKit,morrisirrom/XCDYouTubeKit,ramoslin02/XCDYouTubeKit,donpark/XCDYouTubeKit,morrisirrom/XCDYouTubeKit,itamaker/XCDYouTubeKit,gshaviv/XCDYouTubeVideoPlayerViewController,twobitlabs/XCDYouTubeKit,kumabook/XCDYouTubeKit,c0unt0/XCDYouTubeKit,twobitlabs/XCDYouTubeKit,morrisirrom/XCDYouTubeKit,creationst/XCDYouTubeKit,yjkim/XCDYouTubeKit,gshaviv/XCDYouTubeVideoPlayerViewController,donpark/XCDYouTubeKit,c0unt0/XCDYouTubeKit,morrisirrom/XCDYouTubeKit,c0unt0/XCDYouTubeKit,kumabook/XCDYouTubeKit,donpark/XCDYouTubeKit,0xced/XCDYouTubeKit,yjkim/XCDYouTubeKit,mingsai/XCDYouTubeVideoPlayerViewController,kumabook/XCDYouTubeKit,donpark/XCDYouTubeKit,c0unt0/XCDYouTubeKit,ramoslin02/XCDYouTubeKit