API Model: return dict in get_items()

In some cases, the data model may wish to return supplimental data along
with the items associated with the request.  The get_items() call is now
expected to return a dict with a mandatory 'results' item, and optional
additional items it wishes to be returned to the user.

Change-Id: I81a5ed9d5ecbd647e0fbda021c6dc008f00c9c9d
Closes-Bug: #1356686