Enhancement: Add a "native" JSON serialization

[rpgoldman]

Serializing to JSON through JSON-LD may not be what users with a JSON-based tool chain really want, since it gives a pretty thin layer over RDF, rather than a JSON representation of the meaning of the RDF.

Note that giving a bona fide JSON representation might be achieved by a relatively straightforward extension of SBOL factory (but we would have to modify SF so that it writes its models, instead of just materializing them in the Python image).