Added traffic_ipt plugin based on traffic (now with clean fork)

The traffic plugin didn't work on all computers for me, especially virtualized ones that didn't have the snmp6 files (but did have perfect IPv6 connectivity). To have graphs for those machines, I made a plugin that uses iptables/ip6tables entries instead. It's a bit more work to setup, so you should probably prefer to use the traffic plugin if you can, but if that is not an option, this plugin offers a nice alternative.

This plugin counts both in and out traffic and sums them. I'm not sure what the traffic plugin does, I think it counts only one of them (because when I see both plugins, I see a difference of about a factor 2). I think traffic_ipt is actually more precise.

If I have to change anything to comply with style, please let me know.

This pull request supersedes pull request #247. For discussion, please read that first.