Testing recurrence.

  >>> from icalendar import Calendar
  >>> import os
  >>> directory = os.path.dirname(__file__)
  >>> cal = Calendar.from_string(
  ...   open(os.path.join(directory, 'recurrence.ics'),'rb').read())
  >>> first_event = cal.walk('vevent')[0]

  >>> first_event['rrule']
  CaselessDict({'COUNT': [100], 'FREQ': ['DAILY']})

  >>> first_event['exdate'].dts[0].dt
  datetime.datetime(1996, 4, 2, 1, 0, tzinfo=<icalendar.prop.Utc object at ...>)

  >>> first_event['exdate'].dts[1].dt
  datetime.datetime(1996, 4, 3, 1, 0, tzinfo=<icalendar.prop.Utc object at ...>)

  >>> first_event['exdate'].dts[2].dt
  datetime.datetime(1996, 4, 4, 1, 0, tzinfo=<icalendar.prop.Utc object at ...>)