add delay option to colorbox close

This might not the best implementation, but adding a `closeDelay` option to colorbox will allow for custom closing animations using the callback options (allowing time for an exit/close animation before firing the fade).

Example: I'm using the animate.css `https://github.com/daneden/animate.css` library with the callback options for open and close transitions. See `http://andrewpulley.com/colorboxdemo/example1/` and click on the elastic and fade transition sections.

Thanks!