Move Tags class into new flac_archive package, fix a bug in it, and

Move Tags class into new flac_archive package, fix a bug in it, and
test it.  These stupid scripts will finally start sharing code and
getting some formal testing.

flac_archive/tags.py:
    (Tags): Move here from flac2mp3, and rename ._tags to ._tracks .
    The tag-loading loop from flac2mp3 get_tags becomes the new .load
    method.

    (Tags.get): Return all matching tag data instead of either global
    or per-track (oops).

    (Tags.gets): Adapt to that change, and allow other separators than
    just '\n', which becomes a default.

    (Tags.all, Tags.track): New methods for getting a tag block for
    all tags (whole disc) or just a track, respectively.  To be used
    by fa-flacd in single-file and per-track modes, respectively.

flac2mp3:
    Use flac_archive.tags.Tags.  Use Tags.load in get_tags .

setup.py:
    Note MIT license and install flac_archive package.

test/run:
    Copy test-runner from py-gurps.

test/test.py:
    Load all tests.

test/testtags.py:
    Test Tags class.