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)

It's the hover blue from [the styleguide](http://marketplaceux.github.io/marketplace-style-guides/buttons.html). I'm not sure how that colour was generated though.