[MRG] Use nilearn plotting in examples bis

I think this is good to go. A few things worth noting:

- The scripts generally need z_slice for the plotting function and k_slice, e.g. to figure out the number of detections in the slice. I didn't know better than to use coord_transform. I am wondering whether there is a better way to do this.
- For long titles, using plot_stat_map with a title argument is not going to work out. In theses cases, I had to hand-tweak the figure size and use `display.title` with a carefully chosen y > 1. Better suggestions more than welcome!
- In plot_haxby_mass_univariate.py I had to give up on the ImageGrid and have two separate plots. plot_stat_map colorbar is quite off when specifying an axes parameter.



While I am at it, should I rename all the _unmasked instances by _img?