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 it 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"]
});
```