[TWEAK] Use enum form this._springState
Support function-wrapped transforms in Scene definitions.

Is that the right terminology ("function-wrapped")?

I was trying to do this:

```javascript
    // create the main context
    var mainContext = Engine.createContext();

    // set the perspective (what type of lense?)
    mainContext.setPerspective(1200);

    var initialTime = Date.now();

    // define the main scene.
    var mainScene = new Scene({
        id: "root",
        target: [

            { id: 'background' },

            {
                id: "centerSpin",
                origin: [0.5, 0.5],
                transform: function() {
                    return Transform.rotateY(.001 * (Date.now() - initialTime));
                },
                target: {id: 'logo',}
            }

        ],
    });

    var background = new Surface({
        properties: { backgroundColor: '#25475E'},
    });

        var logo = new ImageSurface({
            size: [400, 400],
            content: '/content/images/logo.png',
            classes: ['backfaceVisibility']
        });

    mainScene.id['logo'].add(logo);
    mainScene.id['background'].add(background);

    mainContext.add(mainScene);
```

but the animation wasn't working. Adding those lines to Scene.js fixed it.