MPRIS music control.

![2014-05-27-184811_1600x22_scrot](https://cloud.githubusercontent.com/assets/1596161/3098845/1b180dc0-e5f2-11e3-8121-c271d95adef8.png)

This will add music control buttons next to the currently playing title and artist.  In order for this to work `now_playing_mpris.c` has to be compiled with the music control functions found in [my pull request here](https://github.com/Lokaltog/candybar/pull/91).