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.
Sorry for missing this last time, can you change the signature of this to `void add_address(struct rt_msghdr *rtm, QueryData& results)`. Note the difference in the type of `results`. Generally, for C++ values, prefer references to pointers. Then you'll have to change `results->push_back(r);` to `results.push_back(r);`