WIP: Manifest caching

This implements support for HTML5 caching that uses a page specific manifest file to control which sources should be cached by the browser.

We need this especially for iPad support since the native caching of mobile webkit isn't usable for "larger" (> 15kb) files. This also should render the combinedModules approach on webwerkstatt unnecessary.

See <http://www.html5rocks.com/en/tutorials/appcache/beginner/> for more infos.