Add a method to fetch all documents by format.
Add a registry to cache and retrieve organisations.
Use OrganisationRegistry to present org titles.

Also wire the configuration for the registry into the app itself,
using a new key in `elasticsearch.yml`. If this key doesn't exist,
the app will not populate organisation titles, leaving them as hashes
containing the slug.
Make webmock errors more descriptive
Option to fetch specific fields for docs by format

When loading the organisations in the organisation registrry we won't need the
indexable_content. This will bloat the data transmitted from elastic search
and cached in memory and we'll never use it.
Only request link and title for organisations.
Don't expand organisations without a registry.

For backwards compatibility, we don't want to change the response format
for organisations until clients support it; then we can enable it in the
deployed config.
Add a comment to be explicit about the cache time.
Merge pull request #95 from alphagov/organisations

Organisations