Calendar use case extracted from Florent's email. This is mostly
materialized in usecase.txt, which is doctested.

- Florent invites Martijn and Bob to a meeting in Room 1. There are 3
  calendars involved, CalRoom1, CalFlorent and CalMartijn. Bob doesn't
  have a calendar but we can contact him by email.

- In the first step, Florent creates a "Tentative" event for that date
  in CalFlorent with attendees Room1, Martijn and Bob in the
  "Needs-Action" state. This (conceptually) sends a Request to the
  other attendees.

- An event in state "Needs-Action" and organizer "Florent" is created
  in CalMartijn and CalRoom1.

- An email with some infor is sent to Bob.

- CalRoom1 is configured to automatically accept an event where it's
  free, so it changes the event to "Accepted". Because the event has
  an organizer (which is a kind of attendee), a reply is sent to
  CalFlorent saying ACCEPTED.

- This changes the participant state of Room1 in CalFlorent's event to
  "Accepted"

- This in turn re-sends the event to previous attendees with updated
  information about Room1's participation status. Maybe we want to
  delay this kind of thing, not that this could mean resending an
  email to Bob which may be too much.

- Later when Martijn goes to his calendar he sees a warning about a
  new requested event in the UI. He reads the event and accepts it,
  this changes its state to Accepted and like above this updates
  CalFlorent, which in turns propagates the status to Room1.

- Later Bob reads his email, and accepts the event. Depending on the
  solution chosen, he could reply to the email and the MTA could
  forward it to Zope, or he could click an URL in the mail. In the end
  the calendar is invormed that Bob accepted the event, so this
  updates its participant status in the event of CalFlorent, and this
  is propagated to other attendees.

- Later Florent reads his calendar and is notified somehow (we'll have
  to find how) that some attendees accepted, and finally turns the
  event into "Confirmed". This is propagated to other calendars, and
  probably a mail is sent to Bob.
