cookie expiration needs to be implemented

[rgrinberg]

I need this functionality myself, but from the source code:

(* We need a non-Unix date/time implementation to support other expiration types *)

What's the reasoning why Unix cannot be used? Is it for mirage compatibility?. As a compromise can we implement max-age instead? I'd like to have max-age anyway.

[avsm]

We can't depend on the Unix module in the core parsing library, as it's also used in embedded contexts such as openmirage.org. However, we could just expose it as a string and leave it to the caller of the library to provide a well-formed date. I've added support for max-age in #73, so that'll be in 0.9.15