Allow clients to connect to Rackspace's "ServiceNet" internal network

Here's a simple patch to allow the `RsConnection` to override the `swift.Connection.storageUrl` and connect to the internal Rackspace network, which is *substantially* faster than the normal public interface.  This is the hacky approach that rackerlabs/python-cloudfiles takes; a better long-term solution would probably be to support the v2 Rackspace authentication mechanism which returns both public and internal URL endpoints and choose from those based on some configuration.