Maintain identity when attributes are referenced

[jacebrowning]

Fixes #47

[jacebrowning]

Converter (immutable) should have to_value/to_data.

Convertible (mutable) should extend Converter and add apply_value/format_data:

• to_value simply calls and returns apply_value on a new instance
• format_data simply calls and returns to_data

During "store", to_data is always called on the converter class.

During "fetch", apply_value is called on the attribute if it's an instance of the converter class and the converter class subclasses Convertible, else to_value is called on the converter class.