Added Support for the RAX-AUTH:authenticatedBy token property

- Added support for the list of authentication types that are returned on the token object from the Rackspace identity service for supporting Domains.  
- Fixed the DomainTestUser to extend from the RackspaceCloudIdentity object to allow for proper testing of the Domain authentication.
- Fixed a incorrect assertion in `UserIdentityTests.TestDefaultIdentity()` method which caused the test to fail