Overlay logo instead of float logo

Instead of floating the logo, we overlay the logo over the login form for all sites that don't have a defined config (i.e. they only work when there's a login form).

Some examples of how this currently looks:

![screenshot 2014-06-30 14 15 54](https://cloud.githubusercontent.com/assets/1097953/3436010/e90a8d8e-009b-11e4-99fa-46504f27a533.png)
![screenshot 2014-06-30 14 15 45](https://cloud.githubusercontent.com/assets/1097953/3436011/e90a86d6-009b-11e4-8acb-624255f93cc4.png)
![screenshot 2014-06-30 14 15 41](https://cloud.githubusercontent.com/assets/1097953/3436012/e90aba98-009b-11e4-8748-5a7c61d035fb.png)