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.
For the block with timeout, you can follow [this example](https://github.com/girder/girder/blob/master/tests/mock_smtp.py#L71).