Plex Support

Support for Plex media server and clients. Fixes bug #16
Note: NowPlaying currently requires a PlexPass. There is no other way to fix that so if you wan't to complain do it here: https://twitter.com/tester22/status/364487438866984961

The music support is limited at the moment and I am not sure it should be included.
Some features like starting items from HTPC-Manager works if the client supports it.