rename ImportTask.path to paths (#89)
legibility refactoring in albums_in_dir (#89)
break up multidisc collapsing tests

Now we can see which aspect fails in bite-sized pieces.
docstring and style fixes (#89)

This uses a simpler way to sort lists case-insensitively. It also sorts the
*files* in a case-insensitive way for consistency.
changelog and docs for #89