WIP : Doc gallery fixes

Fixes the breakages to Nilearn gallery

- makes 5 items per row (changeable if necessary)
- Hovering docstring tooltips added 
- Some js cleanup

Very close to finished - so MRG soon.
Tooltip needs a little bit of rounding off regarding positioning etc but it works well now

[Online build](http://jaquesgrobler.github.io/online-sklearn-build/auto_examples/index.html)