Convert IPv4 mapped IPv6 subnet masks to 4 bytes from 16 bytes (using the last 4 bytes)

[damilola-bello]

@yl2chen
Currently, you can't add IPv4 mapped IPv6 networks.

For example, If you try insert ::ffff:d1ad:35a7/128, a nil network will be inserted. This uses ipV4Ranger (as it is stored as IPv4 address internally), but the mask isn't converted and remains 16 bytes. Hence you have something like this 209.173.53.167/128 instead of 209.173.53.167/32.

func (n Network) Masked(ones int) Network {
mask := net.CIDRMask(ones, len(n.Number)*BitsPerUint32)
return NewNetwork(net.IPNet{
IP: n.IP.Mask(mask),
Mask: mask,
})
}

[coveralls]

Coverage increased (+0.09%) to 91.327% when pulling 6ba34bb on aterlo:master into e434ea7 on yl2chen:master.