Class representing a date range.
A DateRange is created using two dates and can be used to make determinations about dates within the range.
Synopsis:
r0 = DateRange(Date(stdfmt = '2/12/98'), Date(stdfmt = '3/13/99')) # see if a date range includes a particular date if r0.includes(Date(stdfmt = '12/1/98')): print 'yup' # see if the range overlaps with another data range if r0.overlap(DateRange(Date(stdfmt = '12/1/98'), Date(stdfmt = '1/1/2000') ) ): print 'double-yup'