Added four libraries:

* [Pixi Dart](https://github.com/FedeOmoto/pixi) - A port of the pixi.js rendering engine to Dart (from the original author of pixi.js).
* [Play-Pixie](https://github.com/playif/play_pixi) - Play-Pixie is a complete port of the pixi.js 2D rendering engine
* [Play-Phaser](https://github.com/playif/play_phaser) - Play-Phaser is a complete port of the phaser.js 2D animation engine
* [pixi.dart](https://github.com/emergent-design/pixi.dart) - A port of the pixi.js 2D library