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.
Perhaps rename them to `IPv6Gateway` / `IPv6Address`? Slightly longer, but more descriptive.