Add support for ISO8601 time interval parsing

[dparalen]

It would be nice to have the parser return a datetime tuple when parsing ISO intervals

>>> parser.parse('2016-10-04T10:00/2016-10-04T11:15:00')
(datetime.datetime(2016, 10, 4, 10, 00),
 datetime.datetime(2016, 10, 4, 11, 15))

[morrissimo]

still interest in this - most recently, I've had need to parse a duration-only format (ie,P1Y2M10DT2H30M) back into a relativedelta