Adding DocumentFilter::ElaticSearch for publications

This implements an alternative searching and filtering backend for
elastic search.

To use this all the documents need to have been reindexed by rummager
and you need to supply an extra url parameter `use_elastic_search`

There are no tests because this is a prototype. In the production
implementation we intend to use Rummager rather than interfacing
directly with elasticsearch using Tire.
Fix edition unit tests

We don't need to be so explicit about testing the output of search_index
as it's tested elsewhere (in SearchableTest).