Store strong references to async tasks

Reason: https://news.t0.vc/IEUC

main.py

@@ -468,8 +468,8 @@ if __name__ == '__main__':
     sensors.add(SolarSensor('solar', 'Solar'))
 
     loop = asyncio.get_event_loop()
-    loop.create_task(poll_sensors()).add_done_callback(task_died)
-    loop.create_task(fetch_mqtt()).add_done_callback(task_died)
-    loop.create_task(run_webserver()).add_done_callback(task_died)
+    a = loop.create_task(poll_sensors()).add_done_callback(task_died)
+    b = loop.create_task(fetch_mqtt()).add_done_callback(task_died)
+    c = loop.create_task(run_webserver()).add_done_callback(task_died)
 
     loop.run_forever()