PuSH support for RSS feeds

Hi @jonallured,

This is to add PubSubHubbub support for RSS feeds. From what I've seen people use atom tags in RSS using the `atom:` namespace so I think it makes sense to look for it. Here's an [example in the wild](http://www.leancrew.com/all-this/feed/).

P.S. I finally got Feedbin updated to use Feedjira. Great work! I didn't experience any issues and the parser seems more accurate than ever. The code looks cleaner too. Thanks!