Initial commit for the Rackspace DNS API.

@adriancole Do you mind having a look at this initial commit?

I took the stuff you did in your openstack-provider branch and copied it into mine. I only had to make changes to the ProviderModuleExpectTest. When this PR is okay to be merged, I'll delete the openstack-provider branch.

@jdaggett This is the PR I was telling you about with the rate limit domain objects in openstack-keystone. Have a look.