Adding support for the CDN service

This PR adds support for the [OpenStack Poppy](http://www.poppycdn.org/) service. It includes:

* [x] [Code samples](https://github.com/ycombinator/php-opencloud/tree/cdn/samples/CDN)
* [x] [Unit tests](https://github.com/ycombinator/php-opencloud/tree/cdn/tests/OpenCloud/Tests/CDN)
* [x] [Implementation](https://github.com/ycombinator/php-opencloud/tree/cdn/lib/OpenCloud/CDN)
* [x] [Smoke tests](https://github.com/ycombinator/php-opencloud/blob/cdn/tests/OpenCloud/Smoke/Unit/CDN.php)
* [x] [Documentation](https://github.com/ycombinator/php-opencloud/blob/cdn/docs/userguide/CDN)

Where does this limit come from?