Additional events for non-request-reply messages

[clue]

This library currently focuses on the usual Request-Reply pattern used by most of redis' commands. Some commands, however, can reply with several messages per request, notably (P|UN)SUBSCRIBE, WATCH etc.

Removing the requirement for matching replies to requests should be fairly easy. But we still want that kind of functionality for normal requests, so we have to make sure to properly switch states. E.g. once we're in subscription state, additional requests such as GET/SET are invalid and will lead to an error.

Also refs #1.

[clue]

This will be resolved with #23 and #24, so there's no point in keeping this open.