user manager update

- add capability to send notification emails to users on
  creation and password modification (bug#640)
- add success and fail messages for new user addition
- fix bug when modification of username wasn't allowed,
  'Edit user' fields weren't populated
- fix (php5 only?) bug where default group wasn't given to
  a new user if no groups were specified
- added explanatory notes concerning default group and
  password notification emails

This patch adds several new strings to the plugin's lang.php

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