Support caching headers

[CodeMonkeySteve]

Would like to specify caching policy to be included in the response, so that content can be cached by front-end (e.g. varnish).

[ches]

It looks like @rickenharp added Etag and Last-Modified support on his fork, and I'm reviewing that to merge it in -- would that allow the flexibility you wanted via the Rack environment, or does rack-gridfs need additional configurability?

[ches]

Oh, and @bmarini has added an expires option as well.