gdown/gup: add next/previous alias Up and down refer to the customary order in log output rather than concepts; as such, it may be confusing to the user. I for one found them suprising -- they just didn't fit with my mental model. Previous and next, however, imply a clear parent/child relationship. We attroduce the new name as aliases for now.

all: tutorial static/logo-evolve.ico
	sphinx-build . ../html/

tutorial:
	python test2rst.py tutorials/

static/logo-evolve.ico: static/logo-evolve.svg
	convert -resize 36x36 static/logo-evolve.svg static/logo-evolve.ico