Use updated end_time semantics

[avit]

In ice-cube-ruby/ice_cube#99 the meaning of end_time has changed to represent the duration of the occurrence instead of the last occurrence when to stop recurring the schedule. (This is inline with the iCalendar RFC.)

Will need to ensure that end_time is used correctly as it currently is used to terminate the schedule (it should become an until on the recurrence rule instead).