choose best inteface to use.

Instead on broadcasting on all interfaces, zbeacon need to choose the best
interface to use and only broadcast on it.

fix #4
add a way to retrieve the beacon address and broadcast ip

1> {ok, Beacon} = rbeacon:new(9999).
{ok,<0.34.0>}
2> rbeacon:hostname(Beacon).
{192,168,1,11}
3> rbeacon:broadcast_ip(Beacon).
{192,168,1,255}
Merge pull request #5 from refuge/fix/gh4

choose best inteface to use.