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>
Tom Cocca