Caching: Set Expires-header in the future

[commi]

hi all,

turbine sends an 304 if the nothing has changed, but that is still one roundtrip per css.php call, which can add up on some servers an connection.

with an additional expires-header in the (near) future no request would be sent on 99% of the page-impressions. of course only if $debug==0.

it's trivial to add this, but is wanted to discuss it here; i would like to see if there's a major catch to that. if not i will gladly put it in my fork, maybe as an option in the config.php.