Add a basic Ball class.
user
```ruby
class Ball
	attr_accessor :x, :y, :radius, :dx, :dy

	def initialize(x, y, radius, dx, dy)
		setPosition(x, y);
		@radius = radius
		@dx = dx
		@dy = dy
	end

	def setPosition(x, y)
		@x = x;
		@y = y;
	end

	def move
		setPosition(x + dx, y + dy)
	end
end```
assistant