Firefox SignaturePad.fromDataURL fix

Added image.onload.
Without it, firefox failing to load image into canvas. Only after second visit it will properly display.

Example is here 
http://jsfiddle.net/sETn7/

Expected behavior:
browser will display signature

Actual result (FF 24; Windows 8.1 x64):
browser will display signature only on second visit (for example, if you will clean your cache and reload page - browser will not display signature)