Update caching

Add support for Rack::Cache & Memcachier caching based on Heroku recommendations.

Further reading:
https://devcenter.heroku.com/articles/http-caching-ruby-rails
https://devcenter.heroku.com/articles/rack-cache-memcached-rails31
http://railscasts.com/episodes/321-http-caching?view=asciicast
http://hawkins.io/2013/06/caching_object_graphs_with_active_model_serializers/