Cosmetic stuff, plus fixing something I broke.

Just toying around with probably less important things, but I saw we could get album art working, and thought that the tree view for this source could use proper icons.

I also noticed that the new playlist implementation I did had a broken search feature due to how I gave them a type of "google-play-music-playlist" rather than "google-play-music." Fixed that.

Also, threw in a readme overhaul, to make people aware that this plugin only works with RB 2.x.