Allow AuthNS to host mulitple zones

[pinheadmz]

I'm pretty sure this is something BIND can do - if I want to run a service where users can delegate their NS to me and I am authoritative over multiple zones.

With bns, an AuthNS can only have one zone:

bns/lib/server/auth.js

Line 20 in 03b0ea5

this.zone = new Zone();

I may explore a solution that serves an array of zones