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.
This is a great idea! It will need a little work in the general case, consider:

```
global x = "foo, bar = 2", y = 3
```