Figure out browser caching headers

[cholmes]

This is may more of a 'service' level thing, but this repo should evolve to be about the tiler.radiant.earth service. But we should set the proper http headers to cache data. In general I think it's ok to assume that the content of the data won't change, so we can do some longer cache times, perhaps like a month?

This should help browsers perform better on any data the user has seen before. And also sets us up for using CDN's like cloudfront.