Update sml-mode to version 6.1 (Oct 2012)

Emacs sml-mode in this repository has not been updated since 2010, but there are newer versions available. This pull request contains the following versions as downloaded from http://www.iro.umontreal.ca/~monnier/elisp:
* 5.0 (Jul 2012)
* sml-mode-6.0.el (Oct 2012)
* sml-mode-6.1.el (Oct 2012)