Added missing strings from english language files to german language files. TODO: Translate missing strings

- Updated Debian installation instructions.

Fixed: when creating a cp user with modules "mail,dns,sites" and start module "monitor" he gets "LOGIN_REDIRECT:/index.php" after login in.
Added: Better username regular expression check.

- Added dutch translation (many thanks to Hans!)

- Changed shell of website linux user to /bin/false

Added a server module for clients and a mail plugin.