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.
Why make a list and append and then join it? Could we just make a string and append to it? (Or is this by design to save on mem copy overhead?)