Introducing mpdstats plugin to gather statistics from MPD

Please see the plugin docs for details.  This is a *pre-long-weekend* commit, as I'll probably don't have much time this weekend, but want to push this out.  Maybe somebody wants to play with it.

What it does:  Gather *play_count*, *skip_count*, *rating* and *last_played* from MPD by connecting to MPD as a client.

Cheers,
  Peter