Add support for playlists

This change adds a playlists directory parallel to the artists directory.  Within each directory is the name of a playlist, and within that are the songs in the playlist.  The songs are numbers (like tracks would be) based on the playlist ordering.  Because they could be from anywhere, the filenames have the artist, album AND title information.

Things get a bit confusing processing the playlist entries, as Google Music returns them two different ways, based on whether it's a song in get_all_songs() or not.  I worked with the Simon Weber in #gmusicapi to make sure I had the right approach here.

I'm no Python expert -- in fact, this is the most I've ever done in Python -- but I've done plenty of programming in other languages.  So I think the concepts here are sound, and tend to match how the rest of GMusicFS works, but if I failed to do any thing the "proper Python way", please let me know, and I'll correct it.