Added required property to Attribute
Pyflakes + moved DefaultableAttribute to attributes/bases.py
Adding serialization support for DefaultableAttributes
get_declaring_class must also inspect the attr object (not just the name), otherwise it incorrectly returns the superclass if an attribute is redefined.
Completing (de)serialization logic for DefaultableAttributes
Placeholder values for unique, required and default declared on Attributes.

Tests for DefaultableAttributes.
A better get_declaring_class that doesn't require the attr object to be passed in.
merging dynamic-types