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.
turns out debian needs this provider line...so another thing breaking compatibility.  can you set the provider in your platform_family case statement?  Maybe Chef::Provider::Service::Init::Redhat (not sure what works on centos)