Function for setting a default darwin font.

As I am also someone who likes to use Courier when I'm using Emacs, this summer @cassiel showed me some functions he put in his `.emacs-live.el` to in one command change the theme (to either gandalf or cyberpunk depending on the function) and set the font to Courier. I don't remember exactly what the situation was, but if I recall there was some issue with the way Menlo was being set as the default darwin font that was making it difficult for us to change it globally during setup.

I added this code to define a function for changing the default darwin font. It works being called from `custom-configuration.el` (but obviously not `.emacs-live.el` since the function wouldn't be loaded yet) or it can be called interactively.

Don't worry, Menlo 12 is still the default.