POST based JSON-RPC support.

[jmalloc]

This is similar to #11, except that it uses a regular POST request instead of a websocket.

• service-scoped means hitting the /<package>/<service> route, client can invoke any RPC method in that service
• package-scoped means hitting the /<package> route, client can invoke any RPC method in any service within that package
• global-scoped means hitting the / route, client can invoke any RPC method available on the server

We could even allow use of non-JSON encodings with JSON-RPC semantics as I think we'd get this for free with existing content type negotation logic. That said, the only reason I can imagine for wanting to use JSON-RPC semantics with native protobuf encoding is to allow batching over HTTP/1.1.

[jmalloc]

As with websockets (#35), we will only implement the "global-scoped" behaviour, for simplicity's sake.