Use StreamResources with Element.setAttribute

```
Element img = new Element("img");
img.setAttribute("src", createStreamResource());
```

img.getAttribute() should return the URI which also ends up in the client

Attaching the element registers the resource.
Detaching the element unregisters the resource.

Setting the attribute to another value unregisters the resource (no matter if setting a new string or a new resource).

Depends on #386
