add a alien target and a shooter to hit it

I am actually really proud that i was able to figure out how to make the alien move
in every direction randomly.

proposal.md/asteroid_spawner.py

@@ -0,0 +1,15 @@
+from random import randint
+from asteroid import Asteroid
+
+class AsteroidSpawner:
+    display = False
+
+    def play_turn(self, game):
+        width, height = game.board_size
+        game.state['score'] += 1
+        if self.should_spawn_asteroid(game.turn_number):
+            asteroid = Asteroid((randint(0, width - 1), 0))
+            game.add_agent(asteroid)
+
+    def should_spawn_asteroid(self, turn_number):
+        return randint(0, 1000) < turn_number