[Emacs plugin] message -> tern-message

Hi Marijn!

There were people wanting to display Tern messages using `popup.el` (#74), so I thought the first step to allow this would be calling `tern-message` instead of `message`. Then one can redefine `tern-message` to whatever they want. For instance:

    (require 'popup)

    (defun tern-message (fmt &rest objects)
      (popup-tip (apply 'format fmt objects)))))