Superfeedr backend!

Hi!

Stringer is really cool and we're great fans :)
Now, we found that it was kind of pain to run the fetch_feeds job and it could also be quite costly. Even though we could still manually refresh, that's both slow and cumbersome to do. Finally, realtime updates is also a nice feature :)

So, we created the Superfeedr branch which will allow anyone to add some Superfeedr subscriber credentials to get realtime updates. This obviously won't change anything for the users who don't want to use Superfeedr, but is an option to avoid the background job (feed entries are sent directly from Superfeedr to your Stringer instance), as well avoid the hassle of manually refreshing the feeds while propagating them in realtime.

If you want to try it out, you can got to `/setup/superfeedr`, enter some Superfeedr credentials (demo/demo will work) and then wait for feeds to update. [`http://push-pub.appspot.com/`](http://push-pub.appspot.com/)` offers a dummy feed that you can use to test realtime propagation.  

Superfeedr usage is free if you receive less than 10,000 notifications per month (>300 per day), and the pricing is based on the volume beyond that.

PS: we have added tests as well to the code.