Fedora 18 support

Based on the discussion in #145, this enables iptables on Fedora 18 and disabled firewalld.  Determining the persistence command has been changed to compare regexes against the `operatingsystem` and `operatingsystemrelease` instead of `osfamily` since the command is no longer solely dependent on `osfamily`.  Special cases (Fedora 18, Fedora 15-17), go at the top of the `case` statement.  These changes are structured in a way that these changes can easily be applied to RHEL 7 if desired.