[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).

Ouch, code smell: we should be able to deal with nans in the data (because, as you are witnessing, it is often the case in neuroimaging)

I'd be very happy if we could write this example without using nibabel, as an exercise to check that our API is good.