Updated notmuch formula to make use of Emacs.app or Aquamacs.app.

I've read mxcl/homebrew#3568 and I tried to find a different solution to the problem.

The situation is: the emacs version wich ships with OSX is version 22, while the elisp frontend for notmuch (wich comes with the source) need json.el wich has been included in emacs version 23.

My proposed solution is: give user the option to use Emacs.app, Aquamacs.app or the Homebrew version of Emacs, but don't install the lisp code by default. This should be consistent with the current formula (which doesn't install any lisp code), and doesn't add any dependencies.