DOC: contributors logos added to index screen footer

for #2333 

They also show tooltips upon mouse-over (currently just the sponsors name, for in case the logo isn't clear)

Fullscreen:

![screenshot at 2013-08-07 17 26 15](https://f.cloud.github.com/assets/1378870/925255/36808da2-ff76-11e2-9add-f84a9a186f35.png)


Zoomed

![screenshot at 2013-08-07 17 26 23](https://f.cloud.github.com/assets/1378870/925257/3dcf89f0-ff76-11e2-90a5-70117929d689.png)


Feedback welcome