Implements MusicBrainz::ReleaseGroup.search and .find_by_artist_and_title
Adds release fields asin, barcode and quality.
Merge pull request #13 from Applicat/add_asin_attribute_to_release2

Adds release fields asin, barcode and quality.
Merge pull request #12 from Applicat/release_group_search2

Implementation of release group search