Move tufte-latex files to `external` folder

This commit sets up a folder labeled `external` that will contain all of
the files external to the project.  Separating them will make it easier to
attribute the correct license, especially if we need to add local copies
of more packages or fonts.

The commit also renames the `tex` folder to the more descriptive name
`common`.
Move version info into note bibtex field for default CITATION.bib

This commit finishes 6493529 and undoes the 'year' field overloading in the
default bibtex record for the project.
Rewrite paths to tufte-latex files
Set shell in makefile to bash

Setting the shell used by the makefile to bash makes `echo -e` work,
which generates the correct bibtex record for the project.
Add dependency directions to README

This commit adds information about the texlive packages (in Ubuntu) that
are necessary to compile the pdfs.  It would be great to add directions
for Mac and Windows, but those will have to come in the future.
Make minor edits in README
Merge branch 'release-target'