I have stopped working on this API as Spotify have released an official API with a similar use case: https://developer.spotify.com/web-api/web-api-connect-endpoint-reference/
This web server remote controls Spotify Connect devices via HTTP requests.
These are the currently supported methods:
Returns a list of device ID/Name pairs.
Plays, pauses, skips, and returns to previous song.
Gets, replaces, or appends tracks to the playlist. The PUT and POST take
one or more id parameters. Example:
TRACKS="id=2BhU0Hl5OatWiCW93pE2b8&id=731OW49heGHCMrMOREHYlY&id=6zAPaRDoT99ThFtIXUJwhO"
curl -X POST -d "$TRACKS" 127.0.0.1:6767/device_id/tracks