Don't silently ignore older clients

Older clients don't follow the message format, but still produce valid JSON. The result is their messages are accepted and then completely ignored. This PR adds handling for unrecognized messages in the form of an error response to the client, a channel error that closes the connection, and a warning emitted on the server.

This should clear up the problem in strongloop/strong-supervisor#73 once we release a new version of strong-control-channel.

@sam-github I think this might actually warrant a bump of the major version, which will require updating the dependant modules' version specifiers. What do you think?