Add cache-control and expires header in reponse

www.agileplannerapp.com/boards/173808/cards/4800

Cache-Control public allow intermediaries to cache
content. Expires is preferred over max-age or else
we would have to reduce the max-age as we approach
scheduled publishing time of an upcoming edition.

related links:

www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.1
www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.21
@alext: i read parts of that spec, but concluded based on [this article](https://devcenter.heroku.com/articles/increasing-application-performance-with-http-cache-headers#expires). i'll remove that comment.