Adds an active_model endpoint and support for a POST to the model endpoint.
Also adds a stub for a PUT to the model endpoint (to update an existing model),
cleans up the usage of some of the other endpoints to make their usage more
consistent.  This includes modifications to the node registration endpoint to parse
the hw_id, last_state, and attributes_hash values directly from the HTTP request
body (which is assumed to be in the form of a JSON-formatted string) instead of
parsing it from a more generic "json_hash" form field (which contained a reference
to that same JSON-formatted string).  It should be noted that this last change requires
a newer version of the Razor Microkernel (one that will pass this metadata using the
new scheme) in order to work; older versions of the Microkernel will fail to register
with this newer Razor server.