Rename "Name" type to "ArtistName"
find(Artists|Albums) expect (Album|Artist)Name argument, not Text

When looking up artist an albums by name, need to pass AlbumName
or ArtistName types as the argument, rather than simply Text as
before.

I'm not sure if the new or old way is better, as far as API
clarity goes, but doing it this way makes the process of adding
new recordings simpler.
Replace ArtistID with (Maybe ArtistID) in addAlbum

The database schema said that the artist_id column of the albums
table can be null (and indeed this needs to be handled--when
adding a recording with an album tag but no artist tag, for
example), so updated addAlbum to reflect this.
Make addFileToDatabase add artist and album relations if needed

When adding recordings to the database, album and artist fields
are now set, with new album or artist relations added to the
database if not already there.

Needs better error handling--for example, attempting to add a file
that is already in the database will result in an uncaught
exception.