ipv6 support

Hi,
 I have had a stab at adding ipv6 support to Haraka; It needs a bit more review, but I guessed it'd be best to get your thoughts on it before I go too far. There are 3 commits;

* Node will listen on all v6 and v4 addresses if supplied ::
* dnsbl support;
 * I have added a few IP address management functions to utils.js - Is this appropriate?
 * I have added a second dnsbl zone config file specifically for ipv6 bl zones. Is it worth making the original file zones which supply both, and adding a v4 only zone file?
* Adding preferred AAAA lookups to MX resolution.
 * Should there be a config file switch to turn this off?

Cheers,

DDub