0.2.1

• Added Joy.Loader class, which triggers loadStart / loadProgress / loadEnd events.
• Refactor on Engine#currentScene getter/setter to handle with
  Engine#sceneLoader
• Fixed Scene#background(color) method, when scene has no child.
• Omit 'new' when creating Tween instances.
• Use Object.defineProperty instead of defineGetter / defineSetter for
  IE9+ support.
• Use 'canvas' instead of 'canvas2d' when instantiating Joy.Engine with a dom canvas
  element.
• Integrated particle system
• Define behaviour identification as string instead of class references.
• Accept options when attaching behaviours to objects.
• Added Joy.SpriteAnimation class, allowing to define framesPerSecond
  attribute for each SpriteSheet's animation set.
• Fixed issue with sprite loaders. (issue #5)