Less css

Edit : TODO

 - [x] fix body overflow on dasboad
 - [x] fix stderr not red
 - [x] create a test_notebook with most css case 
 - <del>[ ] prepare nbviewer for transition</del>
 - [x] fix or remove print view.


Start to replace css by less.

For now, in browser compilation, kind of **much** better to develop and test as you don't have to restart. 

Start replacing hardcoded value with variable. 

(try setting `@notebook_background`  in `varaibles.less` to a nice light pink, and the rest follows) 

Start removing jquery-ui-class references in javascript. 

Eg, instead of toggling `ui-widget-content ui-corner-all` on cell selection/unselection, replace by a 'selected' class and put the correct css in 
```
.cell{
    &.selected {
        // the code.
    }
}
```
 Browser side parsing should allow things like user only defining css variable in their `.profile/static...` and it should work.

(parsing time is ~+80 ms on my laptop according to less) 

Just for your eyes, what you can do with variables...
![Capture d e cran 2012-12-19 a 21 21 15](https://f.cloud.github.com/assets/335567/22913/bb412fdc-4a19-11e2-9a9b-2700e5b24843.png)

Current style is unchanged of course.