Add support for iptables 'raw' table

The raw table is traversed before any of the other tables and is used to have
packets bypass the connection tracking logic. It has only two chains: OUTPUT and
PREROUTING. See http://www.iptables.info/en/structure-of-iptables.html#RAWTABLE.
Merge pull request #33 from rtkwlf/raw_table

Add support for iptables 'raw' table