Add SoundCloud queuing to v3

This adds in the ability to queue SoundCloud URLs. SoundCloud support was added to MPD sometime at or around 0.17.x, along with a lot of other goodies. For that reason, I'm pulling out the Holman Gist, and properly `brew install`ing the official formula.

There are a few quirks to queuing that I think are attributed to the underlying ruby-mpd library. And, of course, additional quirks because most of this code assumes an actual file path. Rather than use `TagLib`, we'll call out to the SoundCloud API to fetch information. I could add a couple of new database tables to minimize the calls to support this, I guess.

I don't know who's interested in the ping. @holman @thedaniel @maddox @dreww seem like good candidates. Happy  to talk offline for the rationale, e.t.c.