Add support for IPv6

Here's a patch to add IPv6 support.

My bitcoin address is 1KChwCCANvJYWhWyusSZskDGywUjj7n6De in case the people on this reddit post are listening:

http://www.reddit.com/r/bitmessage/comments/1yqxvl/bitcoin_bounty_for_adding_ipv6_support/