Added support for dynamic titles

I've added support for assigning a function to the title option which allows me to do this.

```javascript
$(".fancybox").fancybox({
    title: function(element) {
      return element.attr("title") + "<br />" + element.data("detail");
    }
});
```

I'm actually using Underscore.js templates with more sophisticated HTML, but using simple inline HTML is a quick way to illustrate the concept.