Added functionality to shuffle albums (exposed via the sort submenu)
For this a new class "AlbumList" was added, which fetches all album titles from mpd and offers the possiblity to randomly sort, play and iterate through all albums.
Merge branch 'master' of https://github.com/multani/sonata into albumshuffle
Removed unused code and added improved shuffling of albums by using song ids to move the songs, instead of ranges.
Merge branch 'albumshuffle' of github.com:f0rki/sonata into f0rki-albumshuffle
Simplify the "shuffle by albums" code

* use a defaultdict instead of manually checking for existing values
* use automatically processed "track" attribute, instead of manually
  converting a track value into an integer
* removed unused "sortalbumbytrack" parameter
* removed not so usefull temporary variables and function calls
Remove albumlist module and inline AlbumList code into a function

At the moment, there's no need for all of this, so I prefer to keep things
simple.
doc: updated Changelog
feature: new "Shuffle by Album" feature

Fix: #59