Wishlist: binding for: new Image

Want constructor for javascript Image.

I am currently using this work-around:

```
func newImage() *js.Object {
    return js.Global.Call("eval", "(new Image())")
}

image := newImage()
image.Set("onload", func() {
    //go onLoad(gl, t, textureURL) // call onload handling goroutine
})
image.Set("src", textureURL)
```
