dateutil crashes on `None`

[Dragon Dave McKee]

>>> scrumble.is_date(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/dragon/dev/autonews/src/scrumble/scrumble/dates.py", line 146, in is_date
    d = as_date(inputstring, **kwargs)
  File "/home/dragon/dev/autonews/src/scrumble/scrumble/dates.py", line 142, in as_date
    return dateutil.parser.parse(inputstring, default=default, **kwargs)
  File "/usr/lib/python2.7/dist-packages/dateutil/parser.py", line 697, in parse
    return DEFAULTPARSER.parse(timestr, **kwargs)
  File "/usr/lib/python2.7/dist-packages/dateutil/parser.py", line 301, in parse
    res = self._parse(timestr, **kwargs)
  File "/usr/lib/python2.7/dist-packages/dateutil/parser.py", line 349, in _parse
    l = _timelex.split(timestr)
  File "/usr/lib/python2.7/dist-packages/dateutil/parser.py", line 143, in split
    return list(cls(s))
  File "/usr/lib/python2.7/dist-packages/dateutil/parser.py", line 137, in next
    token = self.get_token()
  File "/usr/lib/python2.7/dist-packages/dateutil/parser.py", line 68, in get_token
    nextchar = self.instream.read(1)
AttributeError: 'NoneType' object has no attribute 'read'