added mpd addon package

Hello,

I have written an mpd (http://www.musicpd.org/) addon for openELEC.  By default, this addon will use the contents of "~/music" as the music library and will output audio to the default sound device (which can be changed by adjusting the mpd.conf in "~/.xbmc/userdata/addon_data/service.multimedia.mpd/config/mpd.conf".

MPD requires the libid3tag package which I have included in "packages/audio/libid3tag".

I would love to see this mpd addon in the openELEC add on repository so that users from all architectures that want to use mpd on their openELEC system can do so without needing to recompile everything.

I have submitted a previous pull request for this.  The previous pull request was not on a named branch, so I have opened a new pull request using a named branch.  I've closed the old pull request: https://github.com/OpenELEC/OpenELEC.tv/pull/2122

Let me know if you have any questions regarding this.  Cheers!