Support pagination on Zerigo DNS zones

For Zerigo accounts with more than 100 domains, the previous zones.all
method was unable to get to records beyond 100. This adds pagination per
the Zerigo API doc with `:per_page` and `:page` parameters. You can then use
count_zones to get the total zone count, divide by your `:per_page`, and
iterate over all pages to fetch all your zones.

https://www.zerigo.com/docs/apis/dns/1.1/zones/index