version 1 of the user service, supporting get_current_user

as a result of the hackathon.

essentially only reference code is committed to the xblock dir
As the commit message implies, this code is only in this branch b/c I pulled it in from `master` via `git checkout origin/master -- xblock/reference/plugins.py`, to stop xblock tests from failing due the unfulfilled django dependency.  All of the diff in this file originated with b63bb0bed4ec736482f5be0fb45a36f3d41818ef, which I didn't author.