Fix error response for "no handler" and "unknown method".
Add MsgpackRPCStream.prototype.setHandler method.

With this a client can set a handler. With this a server rpc can
invoke a rpc at the client:
// rpc at the server
handler.pingpong = function(data, response) {
	// invoke a rpc on the client
	response.stream.invoke('pong', data, function (err, response2){
		// Send response to the client
		response.result(response2);
	});
};