Allow user and email fields of user record to be updated

Fixes #406

* Permit user and email fields of user record to be updated by either
the user or an admin.
* Refactor JSON and XML handling in riak_cs_xm_user to use the
  riak_cs_json and riak_cs_xml modules respectively.
* Add a riak_test module to test the creation and updating of user
  records.
Remove emacs autosave file
Use update_user function in velvet to serialize user updates

Fixes #611

Use the update_user function added to the stanchion client to serialze
user update requests.