Add message length validation to API and fix bugs.

Probably introduced more: really need tests.

* README.md: Update.

* hunchensocket.lisp: Move constants to start of file.
(text-message-received): Rename from MESSAGE-RECEIVED.
(binary-message-received): Rename from BINARY-MESSAGE-RECEIVED.
(check-message): New API function.
(websocket-error): New condition.
(frame): New class.
(read-application-data): New function, read data separately.
(send-control, close-connection): New functions.
(handle-frame): Make more complex, more unreadable.
(handle-handshake,read-handle-loop): Send propoer close connection
messages.

* package.lisp (:hunchensocket): Organize exports.