Enable manual start and stop of stealing volume button events.

2 methods are added to the header which enable the user to decide when to start and stop stealing volume button events:

startStealingVolumeButtonEvents
stopStealingVolumeButtonEvents

Also did a slight modification to the implementation to properly support this functionality.

I needed this functionality in an app where I'm using the volumebuttons to navigate to the previous UIViewController. I only want to steal VolumeButtonEvents when I'm in this one UIViewController.