Constructor = function() {
        if (this instanceof Constructor) {
            Constructor.prototype.initialize.apply(this, arguments);
        } else {
            return new Constructor();
        }
    }
Create class.

When making classes that succeed other classes, the previous class is used as a base with
constructor as default. In order to override the constructor, it is necessary to explicitly
call up the previous constructor to use it.

@example
  var Ball = Class.create({ // Creates independent class.
      initialize: function(radius) { ... }, // Method definition.
      fall: function() { ... }
  });

  var Ball = Class.create(Sprite);  // Creates a class succeeding "Sprite"
  var Ball = Class.create(Sprite, { // Creates a class succeeding "Sprite"
      initialize: function(radius) { // Overwrites constructor
         Sprite.call(this, radius*2, radius*2); // Applies previous constructor.
         this.image = game.assets['ball.gif'];
      }
  });

@param {Function} [superclass] Preceding class.
@param {*} [definition] Class definition.
@static