Added methods to test whether user has prohibited access to the camera

'scanningIsAvailable' returns 'YES' if the camera exists, even if the user has prohibited access to the device.  I tripped over this in my app and decided it'd be nice to know we can't access the camera, even though it exists.  Rather than change 'scanningIsAvailable' and possibly break all current users of that method, I added 'scanningIsAvailableAndAuthorized'.  I also added 'scanningIsProhibited', using it in the other method, and so callers of 'scanningIsAvailableAndAuthorized' can obtain additional information.  While I was at it, I added 'cameraIsPresent'.  Though it is *exactly* the same as 'scanningIsAvailable', it feels more intuitive to me.

Thanks for this excellent Cocoapod.