Made plugins' onNewConnection work and added am onConnectionClosed method...

... to plugins

Some plugins may need to save data for active connections (e.g. chat plugins may need to save a screen name) and/or notify other clients about new or closed connections (e.g. 'xyz has left this chatroom').
This pull request adds an onConnectionClosed handler to plugins and adds code to call a plugin's onNewConnection method, which now also receives the current WebSocketInstance as additional parameter.