Expire cookies that have expired before a request

This implements support for expire. max-age is also probably a thing to want, but for my application this is enough.

Note that there *is* some breakage of backwards compat here – adding cookies manually to `self.cookies` no longer does something sensible (though adding a Cookie header obviously still does). We'd probably want to document that but first I'd like to hear if you guys are OK with that in the first place. The rationale is that the old interface is kinda bad since it throws away necessary information like what was needed to implement this :).

If you wanted some kind of rw interface to cookie values, we could hook up cookies to return something more intelligent than a regular dict, but I personally don't really need that, so I'll do it if it's necessary to merge otherwise I'll leave that for someone else who desires it :).