Reuse Django's FILE_INPUT_CONTRADICTION

floppyforms did define its own FILE_INPUT_CONTRADICTION constant. Because of
this combining the Django core field with floppyforms widget would fail
(both use different objects for FILE_INPUT_CONTRADICTION). The import
from Django core fixes this problem.
Merge pull request #56 from ddanier/issues/56

floppyforms should import FILE_INPUT_CONTRADICTION instead of defining a new object