Refactor feature tests and add combination test

- Split read api tests out into more specific feature tests
- Add feature test for combinations of multiple query parameters

@cpawar
@robyoung
Merge branch 'master' into refactored_feature_tests

Feature tests have moved into sub directories to better organise them.
Merge pull request #39 from alphagov/refactored_feature_tests

Refactor feature tests and add combination test