make promela-mode work on Emacs 24

- removed references to font-lock-default-alist in emacs >=24

- renamed last-command-char (obsolete since Emacs 19.34) to
  last-command-event

- use `define-derived-mode' (do not derive from `prog-mode' yet since
  that is still too new)

- use value of `promela-comment-col'

- do not turn on font-lock unconditionally

- rely on newcomment to do comment indentation

(tested on Emacs 24.3)