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.
So it sounds like I should just have one entry for icons, and link to the 192x192 version with `density: 4`?

Is there any benefit from me manually resizing the 192x192 down to 48x48 and then specifying that resized version with `density: 1`?

Those 144x144 and 152x152 versions were just pre-existing icons, and if there's no benefit from linking to them, I'll just leave them out.