Added  to events API to allow events to be unregistered
Added 'off' to events API to allow events to be unregistered
Merge branch 'unregister-events' of github.com:jrolfs/ZeroClipboard into unregister-events

* 'unregister-events' of github.com:jrolfs/ZeroClipboard:
  Added  to events API to allow events to be unregistered
Added documentation for 'off'
Removed extraneous comment in 'off' method
Merge pull request #84 from jrolfs/unregister-events

Add 'off' to events API to allow events to be unregistered.