Nat chef11

Hey brother, you absolutely saved my life tonight with your addition of nat support to this cookbook.  Unfortunately, it puked on Chef 11:

[2013-03-02T06:22:30+00:00] FATAL: Chef::Exceptions::ImmutableAttributeModification: ruby_block[run-iptables-resources-early] (simple_iptables::default line 34) had an error: Chef::Exceptions::ImmutableAttributeModification: simple_iptables_rule[openvpn] (zuora-openvpn::default line 39) had an error: Chef::Exceptions::ImmutableAttributeModification: Node attributes are read-only when you do not specify which precedence level to set. To set an attribute use code like `node.default["key"] = "value"'

It was a pretty simple tweak to get working so forked and fixed.  Apologies if I followed bad protocol.  First time forking a fork :-)  

Again, thanks for your rock solid work on this.  Really helped me tremendously.  Cheers!