Usage:

    (require 'ac-slime)
    (add-hook 'slime-mode-hook 'set-up-slime-ac)
    (add-hook 'slime-repl-mode-hook 'set-up-slime-ac)
    (eval-after-load "auto-complete"
      '(add-to-list 'ac-modes 'slime-repl-mode))