Allow specification of keys for actors when they are attached to the scene graph

[michael]

Eg.:

scene.add('kevin', new uv.Bar({});
scene.get('kevin').add(new uv.Label({text: 'Kevin'));
scene.get('kevin').p('rotation', uv.PI/2);

[michael]

Is working now.

scene.add(new uv.Rect({id: 'kevin'}))

Or use the declarative syntax:

scene.add({type: 'rect', id: 'kevin'})

To be accessed like so:

scene.get('kevin');

[michael]

Leaving ticket open for discussion