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