Allow data manipulation of AJAX response in Reveal

This adds the possibility to manipulate the data that is returned from the ajax request in the custom success handler.

This is useful if your API/Website returns JSON and you open the modal from custom javascript like:
```
$modal.foundation('reveal', 'open', {
    url: href+'.json',
    success: function(data) {
	console.log('succesfully loaded '+data.title);
        return data.content;
    },
    error: function(jqXHR, textStatus, errorThorn){
        window.location.href = href;
    }
});
```

Here you can now `return data.content;` to let reveal add only the html content part of the response.