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
Hmm. These are all part of the `statistics` ip*tables module which work, so if it is a feature then I would make it a single feature called `statistics`. But since both iptables and ip6tables have the statistics module, there does not need to be a feature for it at all (since features are supposed to describe functionality that only a subset of the providers can do).