Make sure the network interface we grab has a 6 byte hardware address.

On Windows (at least my Windows 7 Enterprise laptop) the first interface returned by NetworkInterface.getNetworkInterfaces().neextElement() is the loopback interface which has no mac address. Therefore, I added a little bit or robustness to the function that got the first mac address. Since its only called once it should not be a performance issue.

I ran the unit tests on windows and ubuntu 13.04.