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.
The user param is never used here, it's only present to provide a consistent API with the other `fileList` methods. We should document that in these sphinx comments in case someone tries to be clever and remove the formal param.