Add Mopidy as a player backend

[Mopidy](http://www.mopidy.com) is an open-source media player, written in Python, with the audio playout managed by Gstreamer. It was an MPD emulation layer, which means it was an easier fit in to the current server setup.

The reason for adding Mopidy was to take advantage of it's many plugins, including [Spotify](http://spotify.com) integration.

It's slower than MPD and currently there is no way of playing the BBC Radio Streams from it, due to an issue of compiling gtreamers ffmpeg libraries on the Raspberry Pi. For this reason, you can select which backend you want for each player in the `config.json` file.