Support msgpack as an event encoding alternative to JSON

[miguelgrinberg]

msgpack support for the reference JS implementation: https://www.npmjs.com/package/socket.io-msgpack-parser.

[zhavir]

what about implementing msgpack as the standard encoding also for the Redis emitter part? Now it seems that you cannot customize that part, then you cannot decouple the socket-io server from the client/emitter implementation due to the fact it uses by default the python pickling. In a complex architecture where each component are heterogeneous, having this strong constraint is bad in my opinion

[miguelgrinberg]

@zhavir your request is separate from this work, so I'm filing a separate issue for it: #755