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.
On second thought, maybe there's something to be said for having a uniform set of formal params across all the model types, so I'm OK with leaving this here. Maybe change it to **kwargs? Not sure on that.