2.0.0 RC2

Added ClientHandler interface. Websocket is now a final class, instead accepting an instance of ClientHandler when constructed. ClientHandler uses the methods handleHandshake() and handleClient() to handle client connection requests and client connections.

When the HTTP server is started, ClientHandler::onStart() is invoked for each Websocket endpoint where the instance of ClientHandler was used, providing the instance of Websocket as a parameter. When the server is stopped, ClientHandler::onStop() is invoked in the same way.