#5 Add PlaylistService support

[Joost Kop]

Implement the PlaylistService defined in http://docs.linn.co.uk/wiki/index.php/Developer:Cara:PlaylistService.
This PlaylistService will let external control points update a playlist stored in the renderer. This way, the control point can be turned off once the playlist is set up, and then it can come back later and make changes if needed.