How about adding d3.image for loading images?

If you want to pre-load images for use in canvas, you have to do this

``` js
var imageObj = new Image();
imageObj.onload = function() {
     // do sth, e.g.
     context.drawImage(this, 0, 0);
};
imageObj.src = url;
```

d3-request could abstract this, so I can load images in a queue etc

``` js
d3.image(url, function(err, img) {
    context.drawImage(img, 0,0);
});
```
