Extended API to provide a methods to find cover art for release groups a...

Extended API to provide a methods to find cover art for release groups as described here https://musicbrainz.org/doc/Cover_Art_Archive/API#.2Frelease-group.2F.7Bmbid.7D.2F

It might be worth considering to provide another Method 
`CoverArtArchiveClient.getReleaseMbid(UUID mbid)`

and to mark 
`CoverArtArchiveClient.getMbid(UUID mbid)`
as deprecated in order to have a more consistent API.

But I didn't want to be so unpolite to mess around with your API so much ;-)