Fix crash when a track isn't loaded on ArtistAlbumWidget.get_songs

If you switch very fast from the 'Albuns' view to the 'Artists'
view, it crashes. Apparently it happens when a track is not loaded
yet, so methods such as get_url() return NoneType which cannot be
converted to 'str' implicitly. It needs to be converted to 'str'
explicitly to be used on logging.debug.
Let's leave 'print' for now - until we implement proper logging
Merge pull request #147 from felipeborges/crash-artists-view

Fix crash when a track isn't loaded on ArtistAlbumWidget.get_songs