`swapChildren` implementation

Simplest solution I could think of -- just made use of `addChildAt`:

```javascript
PIXI.DisplayObjectContainer.prototype.swapChildren = function(child, child2)
{
	if (child === child2) {
		return;
	}

	var index1 = this.children.indexOf(child);
	var index2 = this.children.indexOf(child2);
	
	if (index1 < 0 || index2 < 0) {
		throw new Error("swapChildren: Both the supplied DisplayObjects must be a child of the caller.");
	}

	this.removeChild(child);
	this.removeChild(child2);
	
	if(index1 < index2)
	{
		this.addChildAt(child2, index1);
		this.addChildAt(child, index2);
	}
	else
	{
		this.addChildAt(child, index2);
		this.addChildAt(child2, index1);
	}
}
```

Here's a simple example:

```html
<!DOCTYPE HTML>
<html>
<head>
	<title>pixi.js example 1</title>
	<style>
		body {
			margin: 0;
			padding: 0;
			background-color: #000000;
		}
	</style>
	<script src="../../bin/pixi.dev.js"></script>

</head>
<body>
	<script>

	// create an new instance of a pixi stage
	var stage = new PIXI.Stage(0x66FF99);

	// create a renderer instance
	var renderer = PIXI.autoDetectRenderer(400, 300);

	// add the renderer view element to the DOM
	document.body.appendChild(renderer.view);

	requestAnimFrame( animate );

	// create a texture from an image path
	var texture = PIXI.Texture.fromImage("bunny.png");
	// create a new Sprite using the texture
	var bunny = new PIXI.Sprite(texture);

	// center the sprites anchor point
	bunny.anchor.x = 0.5;
	bunny.anchor.y = 0.5;

	// move the sprite t the center of the screen
	bunny.position.x = 200;
	bunny.position.y = 150;

	stage.addChild(bunny);

	var bunny2 = new PIXI.Sprite(texture);

	// center the sprites anchor point
	bunny2.anchor.x = 0.5;
	bunny2.anchor.y = 0.5;

	// move the sprite t the center of the screen
	bunny2.position.x = 210;
	bunny2.position.y = 150;

	stage.addChild(bunny2);

	setInterval(function () {
		stage.swapChildren(bunny, bunny2);
	}, 1000);

	function animate() {

	    requestAnimFrame( animate );

	    // render the stage
	    renderer.render(stage);
	}

	</script>

	</body>
</html>
```