Update Image.js

[elantion]

Create canvas element seem like not a sync behavior. So we need a callback after create a canvas.

[elantion]

Oh, there is a 'embedded' event trigger after embed. I can use it like this:

image.embed(parentEl);
image.addEventListener('embedded', function(){
        var imageEl = parentEl.getElementsByTagName('img') || parentEl.getElementsByTagName('canvas');
    });

Sorry I didn't read document carefully.