New connected/disconnected hooks, cleanup and README update

* hunchensocket.lisp
(+websocket-terminator+, *websocket-stream*)
(*websocket-stream-mutex*)
(websocket-illegal-frame-type) : Delete.
(websocket-client): initargs for request and stream are now
symbols interned in :hunchensocket package.
(initialize-instance): A trick to have WEBSOCKET-CLIENT's
MAKE-INSTANCE allow other keys.
(client-connected, client-disconnected): New generics.
(message-received, binary-received): Invert order of last two
args.
(call-with-new-client-for-resource): Call new generics.
(with-new-client-for-resource): Pass headers as keyword arguments
when making instances of WEBSOCKET-CLIENT.
(read-handle-loop): Renamed from WEBSOCKET-LOOP.
(handle-handshake): Renamed from WEBSOCKET-HANDLE-HANDSHAKE.
(acceptor-dispatch-request): Always send 404 if client did request
websockets but the resource wasn't found.