new types MBID and ScoreMap

* MBID represents a MusicBrainz ID
* ScoreMap maps MBIDs to musicbrainz Scores (int)
mv Artist* to artist.go + restructured request
mv remaining structs to seperate files
map references instead of MBIDs to scores
restructured annotation search request
restructured remaining search request methods
adds methods to get results with specific scores
hack to pass test. Impl probably broken