Set sensible caching policies

See http://tools.ietf.org/html/rfc7234
Strong, shallow ETags by default

See http://bitworking.org/news/150/REST-Tip-Deep-etags-give-you-more-benefits
for the difference between shallow and deep ETags.
Merge pull request #555 from alphagov/strong-etag

Strong, shallow ETags by default