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

Fixed: Subtemplates can now be given on client creation in remoting (i.e. 10/34/21 for multiple addons)
Changed: template applying moved to separate class
Fixed: force_suexec was not taken from templates

Merged revisions 3049-3051,3053-3054,3058,3070,3074-3077,3082,3086-3088,3091-3092,3094-3095 from stable branch.

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.

FS#880 - field created_at in table clients 

- Update module list of a client only when reseller status is changed.

- Updated Debian installation instructions.

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

Added a server module for clients and a mail plugin.