Make eventsource.html report more info

- Show note about --cgi_path parameter
- Print onopen and onerror event
- Add id to the stream

R=ricea@chromium.org

Review URL: https://codereview.appspot.com/9788045

git-svn-id: http://pywebsocket.googlecode.com/svn/trunk@768 4ff78f4a-9131-11de-b045-6380ec9940d4