Rename 'event' controller to events

Event class name would cause lots of extra typing when using the
AgenDAV\Data\Event class or ugly aliasing (i.e. EventModel or EventObject)

Using 'events' in plural is also better if you think about it as an API
Specify thrown exceptions on docblock

Just a change in a comment, so [ci skip]
Use Events on the event controller

AgenDAV works again!