Hello, this adds a php-fpm service. php-fpm is an alternative fcgi
implementation that is already build with the php package.

About the accounts:
Nginx needs write access to the unix socket of php-fpm. I didn't want to
set nginx as default user for php-fpm in case we add other webservers,
so I added the nginx to the newly created www-data group.

Thank you for reading. I'm looking forward to the revisions,
and happy Hacking!