Ability to get Rackspace ServiceCatalog through Identity API

Rackspace introduced regions for their CloudFiles hosting platform and that means if you want to post to non-default region you need to specify the :rackspace_storage_url to the Fog::Storage constructor.

Fog::Rackspace::Identity.new now returns an instance that has #service_catalog method defined which will return all the services provided for a given API key and that will include all the endpoints available for CloudFiles connections. One can take that endpoint URL and pass it as :rackspace_storage_url to the Fog::Storage constructor to make it connect to the proper region.

Please let me know if there's something I can improve/change to get this applied upstream.