Fixed/Implemented: replaced intval() by $app->functions->intval() in all interface functions due to big number problem in intval()

Added the configuration variable $conf['demo_mode'] in the config.inc.php file. It disables the newly introduced demo mode which limits certain actions in the interface part of ispconfig: the demo mode can be used to run the ispconfig interface part as online demo.

Fixed FS#775 - Client can choose the servers for it's client 

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

Added a server module for clients and a mail plugin.