Web application manifest sample.

@addyosmani @ebidel @PaulKinlan @cwilso (and anyone else who might be interested):

Here's a basic example of a page that uses a `manifest.json` to specify web app capabilities/metadata.

The `manifest.json` is pretty bare right now, so if there's something specific you think it should demonstrate, your feedback is welcome.

You can try it out at https://rawgit.com/jeffposnick/samples/web-application-manifest/web-application-manifest/index.html using Chrome 39+ on Android.