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
Simplify Zerigo DNS list_zones request.
Update docs for Zerigo DNS list_zones request.
Merge pull request #1807 from stormsilver/paginate_zerigo_zones

[Zerigo|DNS] Support pagination on Zerigo DNS zones