Add third parameter to notification callback, so raw request info can be used in the callback.

Allows for handling like this:

superfeedr.on_notification do |notification, feed_id, info|
  topic = req.env['HTTP_X_PUBSUBHUBBUB_TOPIC']
  body_text = info.body
end
Merge pull request #11 from mahemoff/master

Add request info parameter to notification callback