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.
Folders have already implemented that count method, so feel free to add a model_base implementation.

> Alternately, to avoid iterating through the specified resources twice, we
could have the total number of things to delete change as we go (start it
at the number of specified resources, and change it when we get to a folder
to reflect the new knowledge).

I'm fine with that, as long as we change the "title" value of the progress to reflect the current top level element.