Perform callback when the larger image has been loaded

We may want to notify visitors when the zoomed version of an image has
been loaded because downloading the larger image might take a long time.
This adds a callback to cater to this use case. It's used like this:

    $('#your-image').elevateZoom({
        onZoomedImageLoaded: function() {
            alert('The larger version of the image had been loaded.');
        }
    });