Web app and appcache support

Yay!

One thing: I can't really validate the webapp file without deploying it. So you can maybe test that out in production:
https://marketplace.firefox.com/developers/validator and submit http://...../manifest.webapp

If that checks out you can put it on the fx os marketplace! I await to see the app! 

If you're so inclined, you can even use the [mozApp.install API](https://developer.mozilla.org/en-US/docs/Web/API/Apps.install) and have one click install directly from the website!