Added Atom feed processor

The current RSS processor needs a little bit of work (I think), even though it generates a valid feed, some aggregators my feed shows up in don't liike the escaped html that shows up in the ```<content>```.

Now I could have fixed that, but I wanted to have a full on atom feed instead, so I coped the rss processor and made an atom processor:

 - Added Atom feed process
 - Added Atom response extension
 - Call atom response in formatter extensions
 - Added /atom to nancy module
 - this snow.config generates an atom feed rather than rss, but could just as easily do both.
 - atom feed validates on validator.w3.org/feed

The generated feed validates on the w3.org feed validator, but there are still some warnings, which irritate me, so I want to try and get rid of those before I finish this pull request off, but I also wanted to get it out here for a review.