Automatically derive a working base url 

Derive a base_url based on authentication information returned from the server. It collects the first part of the result from connection.get_auth(), optionally overriding portions of it with SWIFT_BASE_URL.

It also ensures that a connection has been established in url(), so as calls to url() have access to a correct base_url.

These patches significantly improve the "out-of-the-box" experience for django-storage-swift.