Add support for getters

[wiseodd]

Getters are declared without a parameter list. The body of the getter is executed when a property with that name is accessed.

class Circle {
  init(radius) {
    this.radius = radius;
  }

  area {
    return 3.141592653 * this.radius * this.radius;
  }
}

var circle = Circle(4);
print circle.area; // Prints roughly "50.2655".