Add HTTP OPTIONS support

Fixes CAA-56
validate_mbid: correct the docstring
Make sure to raise NotImplemented on all HTTP/1.0 OPTIONS requests

Previously, requesting, for example "/*foo" with an OPTIONS request
would return 400 (or 200 if the entity was valid). This wasn't caught by
the test because it tested on a path of "/*" - the only one correctly
returning 501 for OPTIONS requests with HTTP/1.0
Merge pull request #5 from mineo/caa-56-pull

Implement HTTP OPTIONS support
Raise a BadRequest if the image id is invalid

Invalid in this case means it's neither an integer nor "front" or
"back". This was previously supposed to raise a NotFound but as there
can't be an image with an invalid id, a BadRequest makes more sense.
(The MusicBrainz server does also use 400 Bad Request if invalid values,
like invalid includes, are supplied.)

This also fixes CAA-58.
Merge pull request #6 from mineo/caa-58-invalid-id

Raise a BadRequest if the image id is invalid
CAA-60, The query here should be equivalent to the same query in Data::Artwork.

This patch mirrors the changes made to musicbrainz-server in commit a4424e231a6fdb0e18327ef6162015d520d46073.
CAA-60, Revert pointles release_group_meta join in resolve_release_group_cover_art query.
Merge branch 'caa-60'

* caa-60:
  CAA-60, Revert pointles release_group_meta join in resolve_release_group_cover_art query.
  CAA-60, The query here should be equivalent to the same query in Data::Artwork.
Merge branch 'master' into robotstxt

Conflicts:
	coverart_redirect/request.py