The Stacks project

Introduce macros for moduli stacks/spaces

Not sure this is a good idea but at least for the
different versions of Pic it was necessary to more
clearly distinguish especially in statements of
lemmas (bc of the italics that is used there by
LaTeX).

Better handling invertible modules

Of course an invertible module should be defined as a module such that
tensoring with it is a self-equivalence on the category of modules. This
doesn't change a thing beyong the definition, because we usually are
working on locally ringed spaces or topoi where an invertible module is
the same thing as a locally free module of rank 1.

Started tags infrastructure

	new file:   scripts/add_tags.py
	modified:   scripts/functions.py
	new file:   tags/initial_tags
	new file:   tags/tags

Changed the index to at least have an alphabetized list of terms defined
in the project. Thanks Cathy!

	modified:   algebra.tex
	modified:   categories.tex
	modified:   constructions.tex
	modified:   exercises.tex
	modified:   fpqc-descent.tex
	modified:   groupoids.tex
	modified:   homology.tex
	modified:   hypercovering.tex
	modified:   modules.tex
	modified:   morphisms.tex
	modified:   properties.tex
	modified:   schemes.tex
	modified:   scripts/functions.py
	modified:   scripts/make_index.py
	modified:   sheaves.tex
	modified:   simplicial.tex
	modified:   sites.tex
	modified:   spaces.tex
	modified:   topology.tex

Added stuff on Proj of a graded ring

	modified:   algebra.tex
	modified:   constructions.tex
	modified:   modules.tex
	modified:   properties.tex
	modified:   schemes.tex