This adds an 'albums_sort_attr' configuration modeled off of the
'medias_sort_attr'. It defaults to the existing behavior of sorting
sub-albums by directory name. It allows sort by any metadata attribute
stored in the album metadata file.
The existing album sort test wasn't really verifying album sorting
behavior, so I just re-wrote the whole thing to include tests for the
old behavior and new.
This should resolve issue #121
