fishback.createProxy -> fishback.createCachingProxy
Set clientResponse correctly

It should reflect the request that actually went out on the wire, not
the original request.
Move simple.js to caching-proxy.js
Add standard reverse proxy example
Extract constants
Default port to 80
Add example web proxy
Restore clientResponse.url

Move the "real" url to clientResponse.localUrl; clientResponse.url (and
hence serverResponse.url) needs to be the original URL, so that the
cache knows what key to store it against.
Change cache-control header as example
Compensate for url -> localUrl changes
Give more time for response to be cached...