Modify variables to detect websocket timeout errors

[guonaihong]

old--->cannot refer to unexported name websocket.netError

new--->

if err != nil {
    if opErr, ok := err.(*websocket.NetError); ok {
        if opErr.Timeout() {
                //checkout websocket timeout
        }
    }

}

[garyburd]

I do not want to export the netError type because the type will be removed when #474 is fixed.

The supported way to detect a timeout error is:

   if e, ok := err.(net.Error); ok & e.Timeout() {
         // it's a timeout 
   }