Merge initial styles from Art Search into PN

- Add Gruntfile with Sass helpers
- Update `package.json` file
- Update layouts to use local CSS instead of Pure hotlinks

There's still a few TODOs:

- [ ] Update `README.md` to including running Grunt as part of startup
- [ ] Update CSS for the News section of Photonews
- [ ] Use Handlebars helpers from the `yui/pure-site` instead of hard-coding CSS links