Add .dir-locals.el file for Emacs

Emacs 23 and later support directory-local variables:

  http://www.gnu.org/s/libtool/manual/emacs/Directory-Variables.html

Factor has a quite nonstandard c++ coding style convention and there are some knobs you have to tweak in Emacs to get it right. Adding a .dir-locals file to the repo would make it easier to adhere to the convention, and as it's invisible to a normal `ls', it shouldn't be a burden on anyone.