utils: added a uniqyfy_and_sort function that removes duplicates and sorts an iterable
albums: added internal support for multiple propery sorting (issue #138)

Had to do some little changes to make this work on the AlbumModel:
- The album_sort and album_sort_artist were returning a redundant list
  that made ordering kinda quirky. Used the uniqyfy function to make
  them return a more sane result.
- Added a sort_keys dictionary to translate the different sort keys into
  a group of properties to sort by. This may have to change in the
  future to make it more mantianable.
- Changed the sort method to use the sort_keys when creating the sort
  fuction for the sorted collection.
albums: fix dumb name collision on sort_keys
fixed typos
statusbar: start isolating statusbar behavior in a couple of classes
statusbar: finished refactoring statusbar (issue #123)
statusbar: removed extra space on status text
Merge branch 'refactor-statusbar' into order-keys
Merge branch 'order-keys'

Added support for multiple ordering on some order types (to allow a
better subordering inside each type).

closes #138
Conflicts:
	coverart_album.py
	coverart_browser_source.py
	coverart_utils.py