code: function(length) {
        var m;
        if (length == null) length = 1.0;
        if (m = this.length()) {
          return this.scale$(length / m);
        } else {
          return this;
        }
      }
docs: The norm of a vector is the unit vector pointing in the same direction. This method
  treats the point as though it is a vector from the origin to (x, y). Modifies the point in place.
  
  point = Point(2, 3).norm$()
  
  point.x
  # => 0.5547001962252291
  
  point.y
  # => 0.8320502943378437
  
  anotherPoint = Point(2, 3).norm$(2)
  
  anotherPoint.x
  # => 1.1094003924504583
  
  anotherPoint.y
  # => 1.6641005886756874
  
  @name norm$
  @methodOf Point#
  @returns {Point} The unit vector pointing in the same direction as this vector.