Add new DNS seed from bitnodes.io.

I have been working on http://getaddr.bitnodes.io to get a real-time estimate of reachable nodes in the network. The crawler (https://github.com/ayeowch/bitnodes) takes a full snapshot of reachable nodes in the network every 5 minutes.

IPv4 and IPv6 nodes with uptime greater or equal to 24 hours are sampled and a subset of 30 IPv4 nodes and 30 IPv6 nodes are selected randomly with unique ASN and published to seed.bitnodes.io (DNS seeder powered by NSD). The A and AAAA records are given 60 seconds TTL. The sampling is repeated as soon as a new snapshot is taken. This happens approximately every 5 minutes.

The DNS seeder is maintained and operated by myself as part of the Bitnodes project. The server is sponsored by the Bitcoin Foundation and is currently hosted on a dedicated machine based in Nuremberg, Germany.