[MRG] 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).

When can d be None? What warrants that not all ds are None?