add tests demonstrating difficulties with nil dates/dateTimes
handle difficulties with dates and datetimes

- Bail on instantiating a DateType or DateTimeType if adapt_args
  doesn't give us something we can work with

- Don't attempt to setattr(self, name, el) on ComplexType self
  if el is an empty or nil element ... it triggers the first
  change and then bombs in AttributeDescriptor.__set__ when
  we try to set value._position after causing value to be None
handle nils with other attributes
remove unnecessary non-nil attrib check
Merge pull request #3 from jstern/handle_nil_datetypes

Handle nil datetypes