add animations attibute on sprites

Hello,

I wish to use animation on a sprite node.

By animation I mean the property interpolation. Something like

```
var animation = new BABYLON.Animation(
   ...
 );

animation.setKeys( 
   ...
);

mySprite.animations.push( animation );

```

The points is the animations array does not exist on a sprite node. Not a big deal, I instanciate it and it works fine. But I think it will be better to have a homogene API so I add the field.