Improve feed importing robustness

Hey @swanson,

this PR implements changes we've discussed in https://github.com/swanson/stringer/issues/261 that should make feed importing more robust.

Specifically it contains the following changes:
* Adds some feed importing integration specs using an embedded HTTP server
* Changes feed importing mechanism to use story ids to determine wether an article is new or not

I've just deployed this branch to my Stringer instance to see how it behaves in practise. Since the integration tests for feed importing still have only little coverage I would suggest waiting with merging this PR until we've seen how it behaves in practise.

What I haven't implemented yet is using the story url as the unique key for story identification. I think I would prefer seeing first how this behaves in practise and see if using the story id then still is an issue or not.

In any case I'd love to hear your feedback.