Added a new calendarObjectChange event.

This event allows people to specifically hook into CalDAV operations.
It allows you to get direct access to the VObject. Before this event,
you had to use beforeWriteContent and beforeCreateFile, and you were
forced to parse the iCalendar object.

This also makes scheduling a bit faster, because we only have to parse
the iCalendar/jCal object once.

Feature request by @kewisch
`$modified` is passen into emit as a reference and this creates another reference on top of the reference?

I think php will issue a warning on https://github.com/fruux/sabre-dav/pull/538/files#diff-e3faf6194702e826e68b03adfde24bfdR861 because runtime pass by reference is deprecated, AFAIK