Add adapter method to Content API's /for_need endpoint

Making a request to /for_need/need_id.json in the
Content API returns all the artefacts which meet the
given need ID. 

This pull request adds an adapter
method for this endpoint, and a corresponding test
helper.
Merge pull request #96 from alphagov/content-api-artefacts-for-need

Add adapter method to Content API's /for_need endpoint