function registerReqListeners(req, fn){
  req.on('response', function (res) {
    fn(null, res);
  });
  req.on('error', fn);
}
Register event listeners on a request object to convert standard http
request events into appropriate call backs.
@param {Request} req The http request
@param {Function} fn(err, res) The callback function.
err - The exception if an exception occurred while sending the http
      request (for example if internet connection was lost).
res - The http response if no exception occurred.
@api private