Gateway: Log a message->message hash, and put it in message->header

[Athanasius]

To facilitate more easily cross-checking a a Relayed message with what was seen in the Gatewaylog:

1. After the message is parsed into python objects create a hash of the json.dumps(<message component>).
2. Put this into the message header component as, e.g. msg_hash.
3. Log this msg_hash in the Gateway.log

NB: Take care that the json.dumps() produces the same string as the Relay ends up sending out so that it can be reconstructed by a Listener if needs be.