Add an application cache manifest for issue #5.

Everything necessary to run is listed as CACHE entries. Support for the
“json” GET parameter is implemented through FALLBACK entries. Of course,
proxy.php/URI validation can only work when connectivity can be
established with the server.

Tested on IISExpress (and now also on Apache 2.2, web.config/.htaccess are necessary to ensure the correct Content-Type of the jsonlint.manifest file).