This was a vestige from when we used to need the unittest2 library for pre-2.7
compatibility. Now that we require Python 2.7, we aren't using that library
and this indirection wasn't doing any good.
- Add `beets.util.hidden` which adds a `is_hidden` function to check
whether or not a file is hidden on the current platform.
- Add tests for `beets.util.hidden`.