calculated setDragImage offset

[thenetsmith]

Addresses issue #238.

[Stalinko]

It works only in rare cases when dnd-handler is placed right inside dnd-draggable. But it doesn't work correct when between dnd-handler and dnd-draggable are other tags.
Here is more universal code:

if (event._dndHandle && event.dataTransfer.setDragImage) {
    var handleOffset = element.find('[dnd-handle]').offset();
    var elementOffset = element.offset();
    var offX = (handleOffset && elementOffset ? handleOffset.left - elementOffset.left : 0) + event.offsetX;
    var offY = (handleOffset && elementOffset ? handleOffset.top - elementOffset.top : 0) + event.offsetY;
    event.dataTransfer.setDragImage(element[0], offX, offY);
}

[marceljuenemann]

@Stalinko you seem to be using jQuery, which is not a required dependency of this directive and can therefore not be used. I'll take a closer look at this when I do another release.

[Stalinko]

@marceljuenemann, yeah you're right. It was the easiest way for me to calculate absolute offset of an element, however obviously it can be achieved without jQuery (http://youmightnotneedjquery.com/#offset)

[jpuffer]

I made a branch with these changes which does not rely on jQuery. Please take a look! Would love to see this included in the next release.