Add a class that retrieves all artists

Next I'll need a class to actually represent individual artists.
At the moment what is returned is something of the form: 'urn:artist:Bob Dylan' (which is
basically an id).
Once the artist class is written, this can return actual instances.

This has been inspired by Rail's activerecord association methods.
Have the artists class return artist objects
Write songs and song classes and link it with artists and artist

The find methods are currently broken