Recast test/request/atom_feed_test as system test

Includes #181

With respect to #173, specifically (3), I wondered about recasting them as [system tests](http://www.natpryce.com/articles/000772.html).

I think you get good bang for buck with this style: don't need intimate knowledge of rack test, no memory sharing, and you can run them against any API instance.

Working proof [here](https://travis-ci.org/ben-biddington/geminabox).