Make it more obvious how to sign up

We got an email asking how to sign up - that's not ideal. This makes it much more obvious. :smile: 

I think we talked about making the sign in/up form one line, but I fiddled with it too long. I think we can do that in another pass.

![top](https://cloud.githubusercontent.com/assets/65323/4512764/26484d88-4b44-11e4-9a79-7ee06d6942e7.png)
![bottom](https://cloud.githubusercontent.com/assets/65323/4512765/27787890-4b44-11e4-80fa-1d6b6a34fd7c.png)