Changed the way we check for scanning availability

Previously we would use simple methods like `scanningIsAvailable` to check for scanning availability. In iOS 8, the app asks permission to use the camera, which doesn't return instantly and may not return successfully.

To account for this, use the `requestCameraPermissionWithSuccess:(void (^)(BOOL success))successBlock` method to check if the app has permission to access the camera.

This will also gracefully handle iOS 7, which doesn't ask for permission before using the camera.
Updated README with new requestCameraPermissionWithSuccess: method