TODO for Date::Set::Timezone - Timezone/DST code for Date::Set

- verify the context/results for 'days' vs 'hours' in quantize/offset

- floating->union(tz) should return set in timezone (not floating)
  It returns whatever tz the first parameter is.

- t/floating.t - write more tests for tz->union(floating) / floating->union(tz) ...

- fix the 'contains()' code in _tz_change()
   - we don't actually have 'integer' sets 

------ To do later:

TODO: check how does quantize-hours behave under non-integral-hours timezones

TODO: make a special version of new() that accepts a timezone.
    - might need to parse the timezone name in order to make
      correct end-of-DST decisions.

TODO: new() parameters without a 'Z' should be local time,
  with Z should be UTC.

TODO: optimize _tz_change()
   - use offset() code instead of list()

------ Some things I'm not sure yet:

TODO: use a TZ 'id' such that we know 2 sets have the same TZ ?
TODO: Cache tz/stringified version? (uses UTC internally) ?
TODO: open_end option for offset ? - in Set::Infinite
TODO: is_dst( $tz, $ical_utc ) tells if a time is DST or not.
TODO: global TZ, read environment
TODO: make duration() accept the 'utc' parameter ?

