Merge pull request #1 from nvbn/develop

Syncing my fork with base repo
Fixing broken playlist searching.
Icons added to Library tree view and its playlists.
Album art implemented! Closes #23.

While adding songs to the RhythmDB, if a song has a URL for album art
the plugin will add it to the ExtDB.
Updating documentation to address change in Rhythmbox version.

This should hopefully alleviate some problems with people trying to use
this plugin with RB3.

Addresses #39, #45, #46.
Merge pull request #47 from p0lar-bear/develop

Cosmetic stuff, plus fixing something I broke.