flake8 fixes.
Mad refactoring of wald.rdf.

Instead of parsing all RDF at runtime, I've added an intermediate step
which generates a models.json.  The models.json is file is generated
by wald.rdf.models and will be read at service startup by
wald.rdf.factory.  Storing the generated models in .json allows the
models to be read both by server-side python code and client-side
knockout/javascript code.