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
adds license headers
renames *Response types to *SearchResponse
SearchResults now use arrays of pointers
update README & example
fix doc
Update README.md

changes image url
Update README.md

adds GoWalker reference
Update README.md

fix GoWalker link
pretty print struct diffs
more consistency
adds type MBCoordinates
adds SearchPlace method + test