Deal with malformed messages and don't rely on agreement about nicknames

[michielbdejong]

If the agent in the browser extension calls itself 'me', it will send ADD messages with sender: 'me' instead of sender: 'reader' and the monetized-blog example server will break. We should probably either remove the 'sender' field from messages altogether, or make sure its correctness is checked on the receiving side of the channel.