Intelligent deletion of single recurrences

[tobixen]

Ref #35, the best way to handle DELETE-operations on a single recurrence of an event/task having multiple recurrences is probably as following:

• Possibly, remove all attendees first (to avoid them getting notified about the deletion)
• set STATUS to CANCELLED

Possibly, we should rather have a .cancel()-method which may take an optional parameter describing if the attendees should be removed first.

Possibly, recurrence.delete() should raise an error, telling the user to rather cancel than delete a recurrence. Then again, for some app that hasn't consider the complexity of recurrences, it's probably better that the library just do the "right thing" than that the end user gets a weird exception thrown.