[REQS-712] Light-weight holiday source proposal

This is pretty bare-bone at this point. I'm just testing the waters, see if that's the right way to go.
This implementation is going to create a new HashSet with references to every date in the calendar every time `isHoliday()` is called. This seems pretty expensive given that `isHoliday()` is called many times by the analytics. Could we structure the internal state of this class so that this is a simple `contains()`-type lookup?