adding basic :focus style in for buttons in Firefox

![screenshot 2014-03-05 22 19 44](https://f.cloud.github.com/assets/1681963/2341555/343cea1e-a4de-11e3-96a2-9d7b9875a653.png)

based on https://meta.discourse.org/t/focus-on-images-icons-not-noticeable-in-firefox/