Add methods to support the ASNum V6 database

The ASNum database is supported but not it's IPv6 counterpart. These two methods seem to be the minimal interface needed to work with this database (at least, they are enough for my use case):

GeoIP.name_by_name_v6
GeoIP.name_by_addr_v6

I've did some testing and they appear to work correctly (both with hostnames that have public IPv6 addresses and also raw IPv6 and IPv4-mapped IPv6 addresses).