Adding IPv6 network support

While implementing IPv6 support for docker I have noticed that one veth device can only be created with one IPv4 address. I have added an IPv6 address and gateway to network/types.go and extended the Initialize function in network/veth.go to register the IPv6 address and gateway if they are provided.
FYI I'm just a passer-by, reading your PR out of interest, so @vmarmol s opinion is probably more important than mine here, but I thought it was worth suggesting :)