date-iterator

This crate provides two concepts on top of (chrono)[https://github.com/chronotope/chrono]:

CalendarDuration that is able to add months and years (which have varying lengths and thus cannot be represented as seconds)
date_iterators that can be used to iterate over date ranges:
- OpenEndedDateIterator which never stops
- ClosedDateIterator which stops after a given date