[WIP] Glass brain visualisation

This pull request is WIP for now and was only opened to get some feedback, both visualisation-wise and code-wise.

Here is how it looks like atm:
![glass_brain](https://cloud.githubusercontent.com/assets/1680079/4442285/9d705a8c-47dd-11e4-9732-31fc07802367.png)

The code to generate the plots is [there](https://github.com/lesteve/nilearn/blob/glass-brain/nilearn/plotting/glass_brain_scripts/plot_glass_brain.py).

I tweaked these values by hand to have a nicer color range. To be honest, the only use of this plot is to align the brain schematics with the anatomy and eventually hand-tweak the SVG to make them fit a bit better. I don't think it will stay as an example plot.