Add demo with bouncing beach ball

demo.py

@@ -0,0 +1,33 @@
+from turtle import *
+from superturtle.movement import fly
+from superturtle.animation import animate
+from superturtle.easing import easeOutBounce
+
+def nopen(distance):
+    "Move with the pen up"
+    penup()
+    forward(distance)
+    pendown()
+
+def beachball(radius, n_seams=6):
+    """Draws a beach ball centered at the turtle's current position.
+    When finished, leaves the turtle at the original position.
+    """
+    left(90)
+    nopen(radius)
+    right(90)
+    circle(radius)
+    left(90)
+    nopen(radius)
+    right(90)
+
+    for seam in range(n_seams):
+        circle(radius/2, 180)
+        circle(radius/2, -180)
+        right(360/n_seams)
+
+for frame in animate(256, loop=True):
+    x = frame.interpolate(-100, 100)
+    y = frame.interpolate(200, -100, easing=easeOutBounce)
+    fly(x, y)
+    beachball(40)