Add migration text for BITCrashManagerCallbacks
Add support for App Extensions

- Added simple detection method to check wether the SDK runs in an extension
- BITCrashManager will send crash reports without UI (UIAlertViews aren't allowed in extensions anyway)
- Don't start BITUpdateManager, BITStoreUpdateManager, BITFeedbackManager and BITAuthenticator in app extensions