Add initial simple integration test

Hi,

As mentioned in PR #151, I think more tests would be useful - as that PR added some simple unit tests to get the ball rolling, this adds a simple integration test, using supertest and nock to make requests to I/O Docs and mock out a remote service, respectively.

The test does a lot of faffing about moving config.json around - if #146 gets merged, we could look at avoiding that.

Thanks,
Rowan