Added support for RHEL family distros

Notes:

 - Tested & working on: Centos 5.9 & 6.4, Ubuntu 10.04 & 12.04
 - Tested & broken on : Fedora *
   `yum::epel` cookbook does not yet support fedora due to broken
   mirrorlist url:
   "http://mirrors.fedoraproject.org/mirrorlist?repo=epel-#{node['platform_version'].to_i}&arch=$basearch"
   should be:
   "http://mirrors.fedoraproject.org/mirrorlist?repo=fedora-#{node['platform_version'].to_i}&arch=$basearch"
   when platform == 'fedora'
 - Only added 'centos' to official metadata support list, as I did not
   test RedHat