Joda time compatability layer.
RFC 2445 allows mixing of dates, date-times (and periods) within the same
recurrence description. Joda Time makes a clear distinction between dates and
times, so we provide two iterator factories, one for LocalDates and another for DateTimes.
If you need to blur the distinction between
LocalDateIterator and
DateTimeIterator you can
downcast them to Iterator<? extends ReadablePartial>.