Modified to use a callback mechanism.  Backward compatible usage.

I had a need to use an unveil mechanism on a div, not an img tag.

This commit adds an alternate unveil mechanism.  When a div (or whatever) comes into view a callback function is called instead.

Usage is:

$('div').unveil({
    threshold: 200, /* optional, default 0 */
    unveiled: function () {
        console.log($(this));
    }
});

Specifying this unveiled callback will disable the existing handling of the img attributes.  To use the old behaviour, don't specify the 'unveiled' callback.

Existing handling invoked by:

$(selector).unveil();
$(selector).unveil(200);
$(selector).unveil({ threshold: 200 }); /* equivalent to the previous line */