Added support for statistic module

puppetlabs-firewall was having issues parsing rules which utilized the iptables *statistic* module. This fixes that and adds support for 4 new params and includes validation:

- stat_mode
- stat_every
- stat_packet
- stat_prob

Original issue: https://tickets.puppetlabs.com/browse/MODULES-1138
Netfilter module doc: http://ipset.netfilter.org/iptables-extensions.man.html
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