Added support for musicbrainz matching

Unfortunately it sucks.  Using musicbrainz:track:x yields 572 tracks on
my favs, vs 1184 using artist and song names...  Precision on
musicbrainz might be better, but the recall is crap and skimming the
results it didn't look obviously better on precision (still tons of live
album matches).

This thread http://developer.echonest.com/forums/thread/816 discusses
how musicbrainz is not officially supported by The Echo Nest, and why.