Update upload.js

Hi, it seems you use dragexit instead of dragleave. dragexit is specific to Firefox. I propose you this update. May be you should find wise to maintain both events.
cutomize dragenter effect

While i'm here, it would be great to let the dragenter effect to be more customizable. I would suggest using Fx.Morph with a CSS class, but would require 1.4+ mootools. Not sure though.
update css example
Merge pull request #16 from pfz/patch-1

upload popup customization, drag events matching spec