ContentApi#with_tag can request content in groups

This changes the with_tag method in the adapter for the Content API to
send the `group_by` parameter when it is present. 

This also includes a new test helper to stub these requests correctly, 
given a mapping of groups and artefacts.
Merge pull request #124 from alphagov/content-api-group-by

Add Content API method to fetch tag content in groups