Added a demo file showing how to use retro.graph.Graph

Note that you will need to run `mwc update` to update retro-games
to at least 1.1.1.

graph_demo.py

@@ -0,0 +1,41 @@
+from retro.game import Game
+from retro.graph import Graph
+from retro.agent import ArrowKeyAgent
+
+class Player(ArrowKeyAgent):
+    """Player is a subclass of ArrowKeyAgent, so it inherits all of that 
+    class's attributes. However, I'm redefining `try_to_move`: Player
+    only moves if the new position is on the board and unoccupied.
+    """
+    def try_to_move(self, position, game):
+        if game.on_board(position) and game.is_empty(position):
+            self.position = position
+
+g = Graph()
+g.get_or_create_edge(15, 16, 16, 16)
+g.get_or_create_edge(16, 16, 16, 14)
+g.get_or_create_edge(16, 14, 13, 14)
+g.get_or_create_edge(13, 14, 13, 18)
+g.get_or_create_edge(13, 18, 18, 18)
+g.get_or_create_edge(18, 18, 18, 12)
+g.get_or_create_edge(18, 12, 11, 12)
+g.get_or_create_edge(11, 12, 11, 20)
+g.get_or_create_edge(11, 20, 20, 20)
+g.get_or_create_edge(20, 20, 20, 10)
+g.get_or_create_edge(20, 10,  9, 10)
+g.get_or_create_edge( 9, 10,  9, 22)
+g.get_or_create_edge( 9, 22, 22, 22)
+g.get_or_create_edge(22, 22, 22, 10)
+agents = g.get_agents()
+for agent in agents:
+    agent.color = "gray_on_black"
+
+player = Player()
+player.position = (15, 15)
+agents.append(player)
+
+game = Game(agents, {}, board_size=(30, 30))
+game.play()
+
+
+