README.md: Add a `devDependencies` badge

Close h5bp/html5boilerplate.com#113.
Add grunt-uncss.
Base64 encode the background images.

-2 HTTP requests and CSS size is +1.5KB of what it was before grunt-uncss (8.47KB minified without gzip).
Update styles.

* use the shorthand `font` property when possible
* be more explicit with the `background` and `list-style` properties