DT::Incomplete more methods

[fglock]

How about:

  $dti = DateTime::Incomplete->new( year => 2003 );
  $dt1 = $dti->start;  # 2003-01-01T00:00:00
  $dt2 = $dti->end;    # 2003-12-31T23:59:59
  $span = $dti->span;

If there is no 'year', then start,end,span return 
undef.

- Flavio S. Glock