JSON RPC 2.0: fix schema wrt additionalProperties

As JSON Schema doesn't merge schemas, we get in the situation that defining
additionalProperties without having defined any properties or patternProperties
_in a schema_, wherever it is, disallows any and all properties; work around it
in this case by putting dummy schemas for "id" and "jsonrpc", and also for
"result" in the event of a JSON RPC positive response.

See issue #7.