Item copy

This implements an item copy rest call via /item/{} POST.  It adds test coverage for the new functions.  As part of this, I've added a method to the files model to copy a file.  For all of our current three asset stores, I've added code to allow multiple files to use the same data through the use of a repeated reference.
I'm having trouble figuring out how to mock the boto calls with httmock.  I see that there is a python package that is explicitly for mocking boto called moto.  It superficially appears easy to implement.