Movie clip manager

It's nice to have manager that handles different animations of object (like moving/attacking).

#### Example

```javascript
var stage = new PIXI.Stage(0x000000);

var animation = new PIXI.MovieClipManager();
animation.add("left", new MovieClip(/*some textures*/));
animation.add("right", new MovieClip(/*some textures*/));
animation.set("left");

stage.addChild(animation);

animation.play();
animation.set("right").gotoAndStop(1); /* look - chaining! */
```