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.