Improve the postfixadmin migration code:

    - Assign right permissions to domain administrators and
      superadministrator.
    - Correctly prepend the password schema to all admins.
    - Update model classes with the correct varchar length and
      other useful relations.
Merge pull request #609 from carragom/1.1.x

Improve the postfixadmin migration code
Last fixes.

see #609
Documentation update.

see #609
Merge branch '1.1.x'