Added conversion EKRecurrenceRule to an rrule

[Nicholas714]

As EKRecurrenceRule does not have functionality such as getOccurrence() or getDatesInBetween(), this provides an extension to an EKEvent to get the list of rrules for that event.

For example:

let eventStore = EKEventStore()
if let event = eventStore.event(withIdentifier: "id") {
    let rrules = event.rrules
    for rrule in rrules {
        print(rrule.getOccurrences())
    }
}