Add kaminari gem

This gem abstracts the calculations for the offset 
and limit criteria and provides some useful helpers
for returning data about the current paginated set.
Restructure listing_needs_test.rb

Use the same factories for both filtering and all
needs. This means we can use larger sets of fixture
data for pagination without having to account for
the extra three rows here and there.
Order needs by need_id descending

This might not be the correct way to sort the needs,
but let's do something at least so that it looks
sensible and not in natural order.
Paginate needs to fifty per page

Use Kaminari to paginate the need list to fifty
per page.
Give an explicit version to Kaminari
Add links to next and previous pages in the API response
Return information about the result set
Merge pull request #17 from alphagov/paginate-needs