Add support for a exceptionoriginaldate field.

Add support for a exceptionoriginaldate field.

This field tracks the start date of the instance of the recurring event that
an exception is for.  This is needed for sync clients to be able to know
if a specific recurring event instance has been deleted. Since if the exception
starts on a different date (and not just a different time), it's impossible to
match up the events representing exceptions with the list of exceptions in the
Kronolith_Event object.

Now that we have this information, we can get the list of events that represent
exceptions (i.e. event_baseid == the uid of the original recurring event) and
match them up with the list of exceptions in the $event->exceptions. Any left
over dates in $event->exceptions represent instances of the recurring event
that have been deleted.