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.
At least in my tests using the S3 assetstore, when it gets to the delete, there is no key with the name that we expect.  I don't see anywhere in the s3_assetstore_adapter.py script where such a key is created.  Is it created through some other function I haven't noticed?  Is there an actual aws account we have for testing?