Added ability to play and queue Spotify tracks

In order to play spotify tracks, you need to supply some DIDL metadata for Sonos to lookup the Spotify metadata.

You may leave ```parent``` nil, but then Sonos won't be able to lookup the metadata for the track.
```parent``` should be in the form of the album a track is part of

I suppose this method could be easily extended to support Spotify albums, and whole playlists.