Slightly refactor & add tests for API config loading & redis setup

Hi,

I really like I/O Docs, but the PRs I've submitted recently have been made a bit more nail-biting than necessary by the lack of tests. This PR adds some tests for a couple of (relatively trivial) areas of the code, and pulls those bits of code out into their own file.

This is obviously just a small start on a something bigger, but a) there's no reason it all needs to be done at once, and b) I wanted to check you're happy with this as a general approach.

I'd quite like to pull a few more bits out of app.js into their own testable modules (and put a few tests around some of them), and perhaps get some integration tests up and running (perhaps using supertest?) - would you be interested in more PRs along those lines?

Thanks,
Rowan