Revise Entity classes to serialize to valid schemas and consolidate fields #334

Currently none of the citation-related Entity classes that are based on schema.org schemas serialize with the proper @type property that is expected on certain channels. Furthermore, the fields are named inconsistently with 'schema_type', 'at_type', and 'type' all being used by different Entity types to represent the same @type property.

Support may be needed at the Entity base class level in order to allow proper deserializing -> serializing.