Basic song-artis layout
 * If artist is not present in database -> create
 * Songs are linked to artist.
 * Artists are search by mbid(!) BUT name of the artist has to be unique, sorry duplicates;)))
Final song-artist-album-tag model close #4
 * information about songs in downloaded from last.fm by parsing JSON, the first song returned from search is taken as a proper song
 * tags are assigned to artists
 * songs belong to albums & artist
 * mbid for songs & artists & albums is specified
Merge branch 'feature/songs-model'