Use XBlock 0.3
better equality testing primarily for unit tests
Generalize inheritance repr and fn across kvs's and dbmodels
use dbmodel w/ kvs instead of dictionary for model_data
Favor DbModel over DictModel
Move field provenance to xmodule not the kvs
Treat model data as dbmodel not dict
Ensure model_data is a DbModel or compatible and use the appropriate methods to access.
Lms KVS defers to descriptor's default logic