Update fxa banner, simplify text

![screenshot 2014-09-22 09 37 55](https://cloud.githubusercontent.com/assets/211578/4357908/2cefb2ca-4266-11e4-9795-e8e296121d95.png)
320px wide

![screenshot 2014-09-22 09 38 10](https://cloud.githubusercontent.com/assets/211578/4357909/2cf48de0-4266-11e4-8037-ae3807620d46.png)
700px wide

![screenshot 2014-09-22 09 38 25](https://cloud.githubusercontent.com/assets/211578/4357910/2cfa6f1c-4266-11e4-8b36-7672b165dd2c.png)
>1024px wide
We have $narrower-than-desktop