Several fixes and improvements to Rackspace Cloud Files

* Did a refactoring of the action, moved helper methods into a separate class and improved the code.
* Handled the situation when the username and apikey are incorrect, stopping the execution with an error.
* No longer calling the $dz.url method because the URLs that come from Rackspace are already encoded.
* Fixed issue with enableCDN value when running action for the first time.