Use a stemmer when indexing needs.

This lets us match "student" when someone searches for "students", for
example, or "drive" when someone searches for "driving".
Merge pull request #24 from alphagov/stemming

Use a stemmer when indexing needs
Merge remote-tracking branch 'origin/search-needs' (originally 'origin/stemming')

This was originally at 'origin/stemming' however the pull request
merged this back into the (already merged) 'origin/search-needs' branch.

This pulls those accepted changes into 'origin/master'.