Added an overrideable function to AuthBackend which is called at the beginning of UserResource.get_query_set(), which in turn is called by the GET /users/ WebAPI call.  The default function does nothing.  This allows backends to add their own logic to fetch users from an external source and add them to the local user database (or update existing users) before the db query is executed.
I added docs and fixed the doc for another AuthBackend overrideable function while I was there.

All tests pass aside from a few VCS-dependent ones, which are skipped.