Arp vitual table for linux

Implemented #118 with 'system' call in osX (not found other way so far) and reading from /proc/net/arp in linux.

Info shown in vtable is: ip, arp, iface
where arp is the mac address, and iface is the interface this entry was added from.
`sscanf` I couldn't tell you off the top of my head why/if this is scary. But we've been using boost's string manipulation libs for most stuff already. If you prefer a `split` then grabbing the ip/mac by index of the split it could be a cleaner read.