Bitmap fonts

I finished bitmap fonts support. In the example I used Desyrel font from [Starling](https://github.com/PrimaryFeather/Starling-Framework/tree/master/samples/demo/assets/fonts/2x).
Setting font color is currently not supported because of lack of tint.

```javascript
        var loader = new PIXI.AssetLoader(["desyrel.fnt"]);
        loader.onComplete = onAssetsLoaded;
        loader.load();

        function onAssetsLoaded()
        {
            var bitmapFontText = new PIXI.BitmapText("bitmap fonts are\n now supported!", {font: "35px Desyrel", align: "right"});
            stage.addChild(bitmapFontText);
        }
```

Btw. During refactoring loader classes I noticed that it could be problematic if you want to use built-in loaders to load some JSON or XML files. PIXI treats JSONs as spritesheets and XMLs as bitmap font files. I'd vote for changing it but I will require changing API to something like:

```javascript
var loader = new PIXI.AssetLoader({
    font: ["desyrel.fnt"], 
    image: ["1.png", "2.png"], 
    spritesheet: ["mc.json"], 
    json: ["config.json"], 
    xml: ["data.xml"]
});
```