Ruben Kip

Added new: Enum type for python enum.Enum. OneOfValidator for validating value is in given set. Updated: All types, now more strict not accepting None's. (needs Nullable)

After dropping python 2 support in https://github.com/paylogic/halogen/pull/30 there is no more need for six and python 2 compatibility.

Validators `validate` mixed `validate` and `is_valid` approach and returned `True` if valid or raised an `Validation` exception. This PR refactors the validation code to just raising an exception and throwing...