Allow openid to run in stateless mode

If we pass 'storage=None' to an openid provider, it should run the
python openid consumer in stateless mode.

With the default options I was unable to run multiple wsgi workers and do openid auth because it was using a MemoryStore that was not shared among the workers. In order to run in stateless mode, you have to pass 'store=None' to the openid consumer, but the current OpenID provider was preventing that.

MemoryStore is still the default value for storage, but now if you explicitly pass 'storage=None' it will run in stateless mode.