First commit of glass brain related scripts

* svg_to_json_converter.py: exporter which leverages off https://github.com/cjlano/svg and exports the points of the paths in the svg to json
* brain_plotter.py: read from the exported json and plots on a matplotlib axes
* align_svg.py: script with by-hand alignment of the svg on top of the anatomy image
Added a README.md + simple bash script to generate the json files
Removed svg folder
Tiny markdown improvement
First stab at implementing high-level function in img_plotting for glass brain visualisation
Remove stale comment
Add demo for glass brain visualisation
Rename align_svg.py to get the Travis tests to run

nose is trying to import align_svg.py which is trying to do plots and fail with an error like "Failure: TclError (no display name and no $DISPLAY environment variable)"
Additional hack to get the Travis tests to run
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