Update the Plex notifier

The new Plex HT notification API is exactly the same as XBMC's new notification API. It uses the same JSON-RPC methods.

Thus, I've changed the Plex notifier class to inherit from the XBMC notifier class and use its notification method.
Fix the Plex notifier

Previously, the fix would cause the Plex notifier not to work except in test mode or when the XBMC notifier was enabled. This fixes that.
Merge pull request #324 from nightexcessive/patch-1

Update the Plex notifier