- Add support for first() into new methods.

- Redefine Set::Infinite::Arithmetic hashes, to use DateTime
  functions instead of epoch-related internals.
  Actually, it might be better to redo the implementations
  of offset() and quantize()

- Define the new API. 

- Make Set::Infinite::intersects() accept objects as
  parameters (it currently only accepts sets or scalars)
  Such that we can directly check a set against some date.

- Move 'round' procedure from create_recurrence to DateTime.pm

