Move glass_brain_scripts inside plotting

Hoping that the backend magic in plotting/__init__.py will help for the Travis tests
glass_brain_scripts has been moved inside plotting. Also use generic location for json rather than the one on my machine
MAINT: remove unneeded svg_bak folder
Brain schematics inverts its colors with a black background
Add svg bounds into the json document and update brain_plotter accordingly.

Regenerated the json documents.
Revert addition of object bounds margin and only use it when plotting brain schematics
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.
Add optional alpha argument to plot_glass_brain.
Reuse _plot_img_with_bg in plot_glass_brain

Also:
* tidied up plot_glass_brain parameters and documentation
* fix whish -> wish typo
Fix wrong class variable name
Move slice finding code to BaseStackedSlicer.find_cut_coords

cut_coords has been changed from 12 to 7 to mirror what the code was doing in img_plotting