Version 2.0.

• Fields now are descriptors.
• Empty required fields are still validated only during explicite validations.

Backward compatibility breaks:

• Renamed _types to types in fields.
• Renamed _items_types to items_types in ListField.
• Removed data transformers.
• Renamed module error to errors.
• Removed explicit validation - validation occurs at assign time.
• Renamed get_value_replacement to get_default_value.
• Renamed modules utils to utilities.