Rackspace Cloud DNS

Rackspace Cloud DNS

Support Rackspace Cloud DNS

This commit is based on the original pull request by tcocca
  https://github.com/Neilpang/acme.sh/pull/1297

Addtional cleanup was provided by senseisimple in
  https://github.com/Neilpang/acme.sh/pull/1999

This pull request has squashed the changes for review, fixed a minor
(but breaking) problem with the field ordering in the response, and
added documenation per the API guide.

Co-Author: Chris <chris@chrisnovoa.com>
Co-Author: Ian Wienand <ian@wienand.org>