Add IPv6 grok pattern.

Rename IP pattern to IPV4 and add new IP pattern that matches either IPV6 or
IPV4 patterns.

IPV6 regex swiped from http://forums.intermapper.com/viewtopic.php?t=452

It's not the shortest regex out there, but every other regex I found seems to require adapting to the Oniguruma syntax. It seems to work for me on Apache access logs at least.