Add a very basic Rack::Cache around the API.
Fix a rogue lack of newline.
Configure Rack::Cache from a YAML file.

Deliberately not providing a single default rack-cache.yml file, so anyone
trying to run with the cache enabled has to deliberately create or symlink
the config file. Also prevents this running in production without a less
dangerous cache backend configured.
Allow caching in a Redis server.
Explain how to use caching in development.
Disable caching if the config file doesn't exist.

This decouples caching functionality from requiring the related
deployment configuration.
Merge pull request #93 from alphagov/caching

Caching