Ampache API: artist_albums
Ampache API: album_songs
Refactoring, improve test coverage, Makefile

 * Makefile: add php-unit-test-coverage
 * refactor translate empty name for artist
 * 100% test coverage for:
   * album
   * artistMapper
   * artist
   * trackMapper
100% test coverage trackbyFileId
fix coverage report for legacy classes
businesslayer test coverage improvement - 100%
back to 100% PHP code coverage by the tests