[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 an internal function without detailed documentation for other arguments. It says refer to the docstring of plot_img but that doesn't apply to quite a few arguments like bg_vmin, bg_vmax, interpolation, ...

For completeness create_display_fun is the function that takes a display_mode argument and return a display class. Suggestions for better naming welcome!