Check expiration of cached items when serving them instead of computing
an expiration date when first caching them. If the user changes the
caching duration after a request was cached, the new setting will be
used now.
Assets are served from the local cache preferably. They are fetched from
the network if not available. This new addition also enables the "Add to
homescreen" in Chrome/Chromium.
Fix#22.
Lucas Verney