Adding bitmap font support

Adding two classes: an extension to TextureAtlas that parses Starling XML-format bitmap font descriptions (as created by tools like Fontbuilder) and a Graphic that renders them for both buffer and hardware rendering modes. BitmapFontAtlas can easily be extended to support other bitmap font formats. BitmapText has an API that's more or less identical to Text, so it's easy to swap in for existing Text graphics. Tested on Flash and Linux, seems to work on both. Performance seems to be better than regular Text on native targets. Flash rendering could probably be improved as I'm calling copyPixels a lot. Doesn't support word wrapping yet.