Implemented ball-paddle collision.

I decided to have the ball check for collision with
the paddle instead of having the paddle check for collision
with the ball. Either way could work, but it seemed
easier to have the ball do the checking since the
ball will need to respond with a change in velocity
anyway.

paddle.py

@@ -15,11 +15,6 @@ class Paddle:
     def play_turn(self, game):
         if not self.pieces:
             self.create_pieces(game)
-        check_collision()
-
-    def check_collision:
-        x, y = self.position
-        print (retro.game.Game.get_agent_by_name(ball))
 
     def handle_keystroke(self, keystroke, game):
         x, y = self.position