Consider different cases of release's format attribute.

I like to handle different cases of release's format attribute.

E.g. the format of the release with mbid b557ec7f-bfe7-4512-9dce-3a8a7d058189 should be 2xCD as seen on the MusicBrainz website: http://musicbrainz.org/release/b557ec7f-bfe7-4512-9dce-3a8a7d058189

But I got "CDCD".

BTW: this pull request aligns the directory structure of specs with lib if you want.