Delete just one instance of an event series

Added first version to delete an instance of a recurring event. This fixes #98 for me.

What's left to do here?
* Show a list of applied exceptions in the event-edit-view. Currently this is an action that can't be undone. Google Calendar solves it the way, that all exceptions are deleted if you change an event. You can't see the list of exceptions there as well.

I'll soon start using this in production ... so ... would be nice to get this here sometime soon.

How would you make the undoing of exceptions available to the user? Maybe we should add a checkbox "reset exceptions" in the edit-form? Because I can't get a list there :(