Remove duplicate licensing information

Removes the plain-text LICENSE.txt file and moves the LaTeX version to the
top directory.
Add common LaTeX repos as subtrees
Modify README to account for new subtrees

I've basically added a note to the README file warning users
not to modify the remote subdirectories directly.
Refactor textbook to use shared subtrees

* Updates the LaTeX of the main document to use the shared subtree
  repositories.
* Switches to the tufte-handout class and restores title and author
  information.
Quick spell-check of text

Really fast spellcheck, with a lot of decisions about how to handle
the dictionary deferred (there are a lot of technical terms, so
I suppose there should be a shared local dictionary file).
Bump version and update news file
Merge branch 'dev'