Add classNameSize (Bootstrap optional sizes)

A little option attribute for being able to use the Bootstrap optional sizes ("modal-lg", "modal-sm") that we have to add in the modal-dialog div. (http://getbootstrap.com/javascript/#modals-sizes)

Example for a large modal:
```js
bootbox.dialog({
  	message: "Large modal",
  	title: "Large",
	classNameSize: "modal-lg"
});
```