Control Ketting cache via headers

[evert]

I think it will be nice to allow servers to control more features that are typically (now) done on the client Some examples/ideas:

Ketting: no-cache // Never store
Ketting: max-age=3600 // Expire after an hour
Ketting: refresh-on-stale // Auto re-fetch if something marked the item as stale

Some notes on why we can't just use standard HTTP headers: #21

Might also solve problems for #297