Add support for centos (and others in the RH family)

Provided that the epel yum repo is configured (adding yum-epel to the runlist is enough), this cookbook can now install x11vnc and configure it as a service on CentOS.
I see the point for backwards compatibility if you need it. However this syntax is Ruby 1.9 and that is not even the latest version. I can rever this change.