Fix getBlob, getDataURL when image isn't dragged

Initialize the top (and left) to 0px - otherwise top is undefined if the image isn't dragged. This causes a NaN to be passed into context.drawImage in getDataURL and results in a transparent png being returned by canvas.toDataURL().
Merge pull request #5 from KidkArolis/patch-1

fix getBlob and getDataURL when image isn't dragged.