Add app cache in production

This uses `grunt-manifest` to generate a manifest for application caching. Here's the resulting manifest:

```
CACHE MANIFEST

CACHE:
css/app.css
css/octicons.eot
css/octicons.svg
css/octicons.ttf
css/octicons.woff
img/favicon.png
img/mark.png
img/octocat-spinner-128.gif
img/octocat-spinner-64.gif
index.html
js/app.js
manifest.appcache

NETWORK:
*

SETTINGS:
prefer-online

# hash: 9431c072c35371e3a17ad02fc9b36423ed4551eec2e32c15178d85f0d16195e7
```