Rename glass_brain_scripts to glass_brain and brain_plotter to brain_schematics.

Also tidied up brain_schematics a little bit
Move a few files to nilearn/plotting/glass_brain/scripts
First stab at tidying up slicers.py and img_plotting.py

* Add BaseAxes as a common class to cut-based and projection-based Axes.
* Rename do_cut -> transform_2d and draw_cut -> draw
* Add OrthoProjector to display projections of 3D data. Add derived classes
to chose which directions to display.