Compound exhibitions in artworks

Handle change in API. In object resources, the 'exhibitions' property is now a list of objects. Each object has a 'location' and an 'exhibition' property.
Rename method, dictify exhibitions

Rename objects() to artworks().

For some reason when processing the objects resource I decided to take the exhibition hashes and convert them to tuples. Just keep them hashes.
Test changes to exhibitions in artworks