correctly set drag image offset

[adrum]

Fixes #332

Related to #245, #401

#401 didn't work for me with my handler nested within my li. Tested on both FF Chrome.

[kerryj89]

This fixed it for me, thanks!

Edit: Actually, this one makes my table row animate from the top of the screen down to the cursor position which doesn't feel right. I placed my table immediately inside the body element and removed all styling to make sure nothing was conflicting (even though the ghost image exists on a layer of its own unaffected by CSS), and it was still behaving that way.

What worked for me was #401 which worked as described without the animation to cursor issue I had with this solution, a solution which apparently didn't work for adrum, so definitely try out your options if one doesn't work quite right.