implement Paperclip

Add Paperclip::Attachment :image attribute to Artwork via migration,
changing the model, and updating the rake populate tasks.

Also move user-supplied images to public/ and move Artwork-focused step
definitions that were in the artists directory into the artwork
directory.
image upload form

Create view, validations, and controller logic for image upload.
Install DynamicForm gem for form error display. Also clean up artwork
edit page which was still using image_url and add Gallery
Display test to ensure that admin controls do not display when user is
not logged in.