Browse Artists and Albums

Let user browse the existing/cached library.
No network IO calls done during browse.

Directory Structure:

* Google Music
  * Albums
    * Tracks
  * Artists
    * All Tracks
      * Tracks
    * Albums
      * Tracks
Use _lookup_*() calls for browsing

Include AA content for browsing.
Global album list is still without any AA content.
Browsing an artist which is available on AA shows all albums of artist.
fix flake8 errors
Merge pull request #47 from felixb/browse-library