mpdstats: update rating on 'stop' (e.g. last song in playlist)

This fixes #772. When MPD sends a 'stop' event (e.g. by reaching the end of the playlist) the play/skip count and rating will be updated. As a byproduct stopping (instead of pausing) a track will be interpreted as a 'skip'. This sounds reasonable to me, but I'm happy to amend this PR if you disagree.