Add Google Cloud DNS

[stephenplusplus]

Fixes #486

To Dos

• Add support for SPF records in Support SPF records elgs/dns-zonefile#11
• Tests
  • Unit
    • util.exec
    • dns/index
    • dns/zone
    • dns/record
    • dns/change
  • System
• Add GCLOUD_TESTS_DNS_DOMAIN env var to travis (value: gcloud-node.com.)
• Docs

Docs: https://stephenplusplus.github.io/gcloud-node-gh-pages-again/#/docs/master/dns

[stephenplusplus]

Assigned to @callmehiphop for a review, please. Probably easiest to start with the docs, and make sure the API looks sane.

[stephenplusplus]

@jgeewax Since we're trapped on what we can do with system testing (re: the error The 'gcloud-node.com.' managed zone is not available to be created.), here are the options we have that I can think of:

1. Look for a GCLOUD_TESTS_DNS_DOMAIN env var and use it for system testing. This is how it is in this PR currently, and how I've been testing with my stephenplusplus.com. domain (which has yet to run into the issues gcloud-node.com. has)
2. If the env var is set, try to create a zone. If it fails with the error above, try to get a list of zones that already exist, and use one of them to complete the tests that don't involve creating/deleting zones with.
3. If we can't get a zone, or any of the above failed, skip the DNS tests.

It's not exactly pretty to make our tests run like that, but it's possible.

[jgeewax]

I can't find anywhere in any of my projects that have a zone for gcloud-node.com ... I opened a ticket asking WTF is going on... because that's super weird. If you know a project that has the zone created... lmk... I'd really like to use gcloud-node as the demo domain...