Add tufte-latex mirror as subdirectory

Add the EconometricsLibrary's mirror of the tufte-latex project as a subdirectory.
Add latex-shared repo as subtree

Add shared repo and update its ignore list.
Move LaTeX preamble to common file in shared repo

Most of the commands in the preamble are common to all of the LaTeX
documents produced by this project.  Right now it just has a few
workarounds and package loadings, but I expect to put a library of
macros in the common preamble soon.
Group documents into several books

Moves all of the individual LaTeX documents into four separate 'books' and adds
licensing information to each one.