Trying a new layout to prevent flickering on mouse hovering

Hi Lea,

We talked about "problems" with mouse hovering and the layout of the boxes in #10.

I took different tries and thoughts on the problem. It seems that adding markup in not an option to achieve what we want.  

I'm just sharing my thoughts out here. There's probably naming stuffs etc to improve and refactoring to do. For example we don't really need classes on that new invisible background div...

Please let me know what you think about it ?

As you advised me, I'm prepared for your perfectionism ;-)