Add API method to get artefacts for a given need_id

Offered as an option. Not sure if this should be its own method
or a filter on the /artefacts method
Restrict /for_need method to only return live items
Allow authorized users to get all items for a need_id

Authorized users should be able to get all items, regardless of their current state.
Merge pull request #131 from alphagov/artefacts-for-needs

Artefacts for needs