Add stream relay support

Groovebasin currently supports playback of local files only. MPD supports playback of live streams over HTTP. For Groovebasin to work as a replacement for MPD, HTTP streaming should be implemented.

Groovebasin should go one step further and support HLS streams. There is support in [libav](https://libav.org/general.html#Network-Protocols) for this, which I presume is how VLC supports playback. As libgroove uses the same library, I am optimistic that this could be implemented in the same way.

**Some background to this request:**

I'm currently working on a project to [build radio prototypes](http://radiodan.net/), using open-source software and boards such as the Raspberry Pi. We're currently using MPD, with the same love/hate relationship you highlight in your [blogpost](http://andrewkelley.me/post/quest-build-ultimate-music-player.html#rejecting-mpd). We would love to replace MPD with groovebasin but a lack of http streaming makes it a non-starter. The BBC streams all of its content over HLS, so finding a remotely controllable, light-weight player, that supported HLS would be a dream come true.