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

NETWORK:
*

# hash: 7c3b8186844277179b64a62972f33bbd56496ee5ef72611884420a7a589d32f9
```

This is currently deployed to http://gh-notifications.heroku.com/