Searches can specify a sort field and order

This will let us add a "sort by date" feature to (government) results on
gov.uk/search

The API copies advanced_search, and relies on the Rack::Utils
parse_nested_query behaviour, common in Rails apps:
http://rdoc.info/gems/rack/Rack/Utils.parse_nested_query
Use flat params for sort and order

This decouples us from non-standard Rack::Utils nested_query_params, although
it loses the ability to specify multiple sorts.

I chose "sort" for the fieldname from reading wiktionary:
  5. (computing) An algorithm for sorting a list of items into a particular
     sequence.

and "order":
  1. (uncountable) Arrangement, disposition, sequence.
Document a default
Merge pull request #132 from alphagov/searches_can_specify_sort_order

Searches can specify a sort field and order