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.
I think we need special handling here for folders. If progress=true, we should count their size recursively just like we do for the folder deletion endpoint rather than just counting each one as the same value. If progress is false, we don't need to count anything.