account rename support

Taken verbatim from: http://www.mediawiki.org/wiki/AuthDrupal

* Users are matched based on usernames only. If you rename a user on the Drupal site, they will not be connected with their existing login in Mediawiki. If you delete a user and create a new one by the same name, Mediawiki will connect the new user with the old account of that name on the wiki side.

We should probably identify based on the Drupal UID.