[google|dns] Add initial support for Google Cloud DNS.

- some common plumbing
- requests for creating, deleting and listing managed zones, with tests

One thing we should discuss is the test domain in tests/google/requests/dns/managed_zone_tests.rb
For the test to pass in non-mocked mode, the developer who runs the tests needs to confirm its ownership with Google. This implies that it needs to be configured for every developer individually.
Alternatively (and this is what probably should be the default) is to disable most (if not all) the Google DNS tests in non-mocked mode.
Done.