[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).

This is a kludge that is only used to be able to get the alpha to percolate from img_plotting.plot_glass_brain to brain_glass.plot_brain_schematics. Eventually they get passed to the matplotlib patches of the brain schematics.

To be honest I am not sure we need alpha. The plots do look better with alpha=0.7 (which has been used for the most recent snapshots) but maybe we can achieve a similar effect by just tweaking the color of the lines in the svg.

*alpha=0.7*
![glass_brain_alpha_07](https://cloud.githubusercontent.com/assets/1680079/5279213/770ce332-7ae2-11e4-9b85-d9c94fb7d80d.png)

*alpha=1*
![glass_brain_alpha_1](https://cloud.githubusercontent.com/assets/1680079/5279241/d802030c-7ae2-11e4-8b05-da056d89f428.png)