Add onZoomedImageLoaded callback.

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.');
        }
    });