Add a .dir-locals.el to theories

Thanks to Mike Shulman for the internet
(http://stackoverflow.com/questions/4012321/how-can-i-access-the-path-to-the-current-directory-in-an-emacs-directory-variabl/11474657#11474657
and
http://syntaxexclamation.wordpress.com/2011/08/11/set-the-root-directory-of-a-project-in-emacs-with-dir-locals-el/)
and to google (for
http://stackoverflow.com/questions/3730742/concatenate-string-in-elisp
and
http://stackoverflow.com/questions/2290055/concatenating-strings-in-muse-project-alist).