Add method to GdsApi::NeedApi to fetch all needs

This request returns a response from the Need API
which is compatible with GdsApi::Response::ListResponse.

This also includes a test helper method `need_api_has_needs`
for use in the tests here and in apps which use
the adapter.
Delegate additional ListResponse method to results

The `to_ary` is required for rendering collection
partials correctly using ActionView::PartialRenderer
Merge branch 'master' into list-needs
Merge pull request #83 from alphagov/list-needs

Add method to `GdsApi::NeedApi` to request a list of needs