Update styles

* Use the shorthand `font` property when possible.
* Be more explicit with the `background` and `list-style` properties.

Close h5bp/html5boilerplate.com#115.
Remove `main.css`

File is not longer needed.
Add grunt-uncss.

This saves ~30% of the final minified, uncompressed CSS.

Before: 19.97 kB → 7.07 kB
After:  19.97 kB → 15.4 kB → 4.88 kB