TODO for DateTime::Format::ICal

- When given an EXRULE: or RRULE: part, parse_recurrence should
  be able to fix this:

"The iCal spec requires that "dtstart" always be included in the
recurrence set, unless this is an "exrule" statement.  Since we don't
know what kind of statement is being parsed, we do not include
C<dtstart> in the recurrence set."