Search on the need_id field in addition to _all

This changes the search query to include the need_id field. To do this,
the query has been changed to a 'multi_match' query, which now performs
an 'or' search across the "_all" and "need_id" fields.
Merge pull request #36 from alphagov/search-on-need-id

Search on the need_id field in addition to _all