Shapes

These commits improve Sprite support.

(1) Adds a "dot" sprite shape (e.g., 'red dot')
(2) Changes the "pointer" shape so that the origin is at the indent in the back, for more intuitive turning.
(3) Simplifies the "Sprite" constructor so that it is more similar to the Turtle constructor, except that it defaults to a square sprite shape.
(4) Allows you to say new Turtle(num) or new Sprite(num) to create a sprite with <div>num</div>.