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.
can you please do this with a std::string instead of mallocing a char*?