I suggest we rename this to `user_id`. (I'd actually much rather call it simply `id`, or `subscriber_id`)
We should cache the result, and invalidate it any time the email address is modified, otherwise we have to do two net requests in `#api_update`.

Finally, I don't think it makes sense to return 0. I think nil is a more appropriate response when there is no user ID returned. Please weigh in on this, because we'd have to modify some of the other code to support this change (or would we?).