Add IPv6 City lookup function and expose netmask fields

[horgh]

These commits do a few things. I'd be happy to split them into separate PRs if you prefer.

Primarily:

• Adds the ability to look up City records in IPv6 databases
• Exposes the netmask field of GeoIPRecord
• Exposes the netmask field of GeoIP. This is useful if we don't find a record and so can't use the netmask from GeoIPRecord
• Expose toggling the Teredo flag
• I added a bit more validation as well.

Thank you!

[horgh]

Ah, I see #29 also adds a way to do IPv6 lookups, though in a bit different way.