Display multiple metadata values

In order for documents to have metadata values with multiple options
present, this commit does the following:

- in `abstract_document` map over all the values and find the label for
  each, then pass them through as "Item 1 + N others"
- refactors the tests to reflect that tag metadata is now always in Arrays
Merge pull request #42 from alphagov/display-multiple-metadata-values

Display multiple metadata values
Add mustache dependencies and include in application.html.erb

- shared_mustache
- include js in application.js
- include mustache template handler in layouts/application
Add a ResultSet presenter, tests and mustache template

This will build the ResultSet hash that we'll pass to mustache. The result set presenter takes a finder and then outputs only the information used by the result set mustache template through the .to_hash method

This commit includes:

- Initial stab at tests
- Initial stab at interface between ResultSet and mustache
Use ResultSetPresenter
Move tests + methods from application_helper

Because the presenter is now handling the data formatting we can remove the methods and tests from the application_helper and put the methods in the result_set_presenter
Move result presenter into separate presenter

Remove the result specific handling stuff and put it into a separate presenter.
Add a to_hash method on Facet.rb

Takes values that have been selected on the facet and returns them as a hash for the presenter
Add selected facets hash to facet_collection

Add a method to get selected facets in this facet collection as a hash and update tests

The hash returns the preposition, key and selected values hash which contains all the selected values for this facet. The returned hash contains all the data used by the ResultSetPresenter to form a sentence summarising the search results for the mustache template
Add a to_hash method to resultset

So that the presenter can present this to the mustache template for search results
Rename selected values method

I kept tripping over this so I've renamed it which is a bit more self explanatory I hope
Swap in mustache to finders/show.html.rb

Use results.mustache instead of results.html.erb
create search result presenter + specs

To make the ResultSet presenter a bit lighter, moving result specific behaviours into a new presenter. Adding tests.
Delete unused templates
Push date formatting onto SearchResultPresenter/template

This makes the templates a bit more complex but stops us having to include all the helpers in the presenter which is a smell. Updated tests to follow.
Move html building stuff into helpers
Remove helpers that we're no longer using
Add pry gem for debugging
Update search_result_presenter tests

Add new tests, remove old tests that are testing deleted code
Delete unused code from facet.rb