App will log exceptions if in prod mode.

If in development mode it will log in the default behavior.
Merge pull request #13 from alphagov/feature/logging

App will log exceptions if in prod mode.
@wip add querist
@wip add failing test for requests
Add request promise

- wrap up general request logic in request-promise
- add in chai-as-promised to check our promises
Fixed linting errors.
Add require-subvert

Allows us to mock dependencies that are required on a function that is exported on a module.
Add lots of tests for request-promise
Fixed up testing for querist

Use require-subvert to test the exported function