include artwork permissions in the dataset

[tacman]

Because some of the artworks in the collection change status on whether they can be included in this dataset (or our permissions for the image)

From #29

I would love to have a license code (e.g. cc-0) in the Artwork data, so if I'm working on something with images I can filter out anything that doesn't have the right license.

The documentation just says that images aren't included, but this comment makes it seem like some images might be available already.

[tacman]

On a related note, I requested an artwork from artres.com back in February and haven't received a response.

It's not urgent, as the whole purpose of the request was to see the workflow and how long it took.