Condition type classes: only expose (de-)serialization methods (DER, URI, JSON)

[sbellem]

From @r-marques in https://github.com/bigchaindb/cryptoconditions/pull/67#issuecomment-310113354:

I don't think we should expose methods like to/from asn1_dict/asn1_json since they are on the spec and are specific to different asn1 implementations. We should only expose binary, uri and json. Ideally we would have a json schema that is part of the cryptoconditions spec.