Register button on devhub (bug 1095659)

WIP: still some touchups to do.

![sign-in-up mov](https://cloud.githubusercontent.com/assets/211578/4961774/fe1abd78-66d2-11e4-9fd9-17f66867526d.gif)

I'm not talking about for all the other anchors. the whitespace is adding to the padding between the anchors. load https://marketplace.firefox.com/developers/ and add `font-size: 0;` to `.devhub-links` in your favourite browser's Dev Tools.