Fix is_date_field for simple types

Having a simple type returned from e.g. a hybrid_property, an
`AttributeError` would be thrown from `is_date_field`. Fixed by checking
for the `property` attribute before accessing it. Added test case that
provoked the error prior to the fix.
pep8 fixes
Merge pull request #220 from logandk/hybrid_setter_error

Fix is_date_field for simple types