Add Travis CI support

Ref: https://travis-ci.org/
Add `grunt-html-validation` plugin

Additionally:

* Add a Grunt `test` target which runs the needed tasks.
* Fix existing validation errors.
README.md: add a devDependencies badge.
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