[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).

> The thing is that draw actually draws some 2d data. I could rename it draw_2d
> to make it more explicit.

Yes, that's a good suggestion.

> As a side comment I also plan to go over the file and remove the cut*
> occurrences where easily doable.

Good idea. Thanks!