Unit tests and bug fix for aliases (#102)

samdoshi@70b5b3ad4435516b5ef52ad611190b8efd9cdf4f is a small bugfix (introduced in SHA: 9f5926a026713b6f0e6600f4c94f96e3ad8f3661), without it only the first language in _languages_ is considered.

samdoshi@8394619b994e04d908faf98dbc8ab6c5de252f41 adds unit tests for aliases (via ArtistFlatteningTest and _flatten_artist_credit)