Use the getMessage() function as intended

This is the intended way to get the message, not our hacky way of getting the internal variable.
This way, python-openid will set all the missing fields for us automagically.
Isn't that nice of it?!
Re-add server_url

We could make python-fedora use the official endpoint url variable, but backwards-compat....
Merge pull request #108 from fedora-infra/use-python-openid-as-intended

Use the getMessage() function as intended