Add DateTime.startOfMonth(), Period.findPrev()
- Add DateTime#startOfMonth() which returns the date truncated to the first day of the month at 00:00:00
- Add DateTime#startOfYear() which returns the date truncated to the first day of the year at 00:00:00
- Add Period#findPrev() analogous to findNext()
- Allow count parameter <= 0 in Period#findNext() / Period#findPrev() i.e. findNext(dt, -1) === findPrev(dt, 1)