Merge branch 'entity-proxy-fields'

* entity-proxy-fields:
  Implemented EntityForm, which knows how to deal with AttributeFields and RelationshipFields on the underlying model. To be consistent with Attributes, Relationships now support null values. This changes the schema.
  AttributeFields and RelationshipFields now store their changes and do not commit them until their model has been saved.
  Initial implementation of AttributeField and RelationshipField. These objects are not actual Fields, but rather they simply add descriptors to Entity subclasses which provide transparent access to specific attributes and relationships.