Checked items now work

This address issue #118.  The download and delete actions work as expected.  Internally, there are two new endpoints: resource/download GET and resource DELETE.  The download can actually get item, folder, user, and collection resources, and the delete can work on any model that has a remove method.
Maybe "girder-folder-metadata.json" and "girder-item-metadata.json" would be even more descriptive.