Refactor: definition of message_for_client

When we want to talk to the client we send a JSON string. This string
must always contain a "type" value so that the client can handle it
(e.g. "order" or "system")
Refactor: improved messaging between server and clients

We now have a convention. "Order" is something that always goes from
server to client, and "Response" is something that goes from client to
server.
Update to README.md
Refactor: simple function renaming