FIX: websocket_handle_event/3 signatures for websocket_open and websocket_close in elli_example_websocket.erl

In elli_example_websocket.erl:
FIX: websocket_handle_event/3 signatures for websocket_open and websocket_close
ADD: how-to comments in websocket_handle/3 for simple echo server implementation
Merge pull request #1 from n-o/master

FIX: some websocket_handle_event/3 signatures