Improvements to contrib/julia-mode.el (Emacs mode)

Add functions to execute Julia code in Terminal.app on Mac OS X.

Improve font lock.

Function julia-documentation based on ESS’s Julia mode plus fix.

Set comment-add to 1.

Add key bindings.

Add logic for auto-mode-alist.
Well, that makes handling and installing more involved, and I am not sure if this is worth it for a few lines of code.

I wrapped the key bindings in (when (eq system-type 'darwin) ...), and we can do the same for the OS X specific functions. Then one won't see any of this on another system.

Or we could create a julia-mode-mac-os-x.el file, if people feel it is necessary to separate the codes.