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

Why? When working with release groups its much easier to find cover art when you can just find by the group's MBID instead of iterating over the releases and querying their covers.

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 ;-)