`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~~ more thorough example, which you can test directly by [checking out this branch](https://github.com/namuol/pixi.js/tree/swapChildren-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");

	var i, i2;
	var bunny;
	for (i=0; i < 10; ++i) {
		// create a new Sprite using the texture
		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 = 150 + i*2;
		bunny.position.y = 150;

		stage.addChild(bunny);
	}

	for (i=0; i < 10; ++i) {
		for (i2=0; i2 < 10; ++i2) {
			console.log('Swapping ' + i + ' with ' + i2);
			stage.swapChildren(stage.children[i], stage.children[i2]);
		}
	}

	setInterval(function () {
		var i1, i2;
		i1 = Math.floor(Math.random()*stage.children.length);
		i2 = Math.floor(Math.random()*stage.children.length);
		console.log('Swapping ' + i1 + ' with ' + i2);
		stage.swapChildren(stage.children[i1], stage.children[i2]);
	}, 100);

	function animate() {
	    requestAnimFrame( animate );

	    // render the stage
	    renderer.render(stage);
	}

	</script>

	</body>
</html>
```