rougeth#175: Changed Bottery.stop behavior.

This patch fixes two problems in the Bottery.stop behavior:
1. self.session.close() was not awaited;
2. self.loop.close() causes an exception "Can not close running loop" in
some cases.

bottery/bottery.py

@@ -104,5 +104,5 @@ def run(self, server_port):
         self.loop.run_forever()
 
     def stop(self):
-        self.session.close()
-        self.loop.close()
+        asyncio.gather(self.session.close())
+        self.loop.stop()