Rebuild calendar algorithm to use Google Calendar API

The !next and !nextfan commands will now pull the list of events from
the Google Calendar API, which should make them a lot more accurate
going forward.

Also... added a !nextfan command.

Remove icalendar from dependencies, as we don't need it any more. Also
remove the version restriction on python-dateutil, as we don't need that
hack monkey-patch for dateutil.rrule any more.