#17 link recurrence exceptions to their original event

[Sebastian Schneider]

android keeps some database field to link a recurrence exception to the original event (RRULE), currently these entries are left open, since at the time of storing the event, we cannot be sure that we even know of the RRULE-event, or if we have in its actual version. also, deleting the RRULE-event should also trigger the removal of the exception events (unless android handles this already)