asyncio_slow: Add callback example from docs.

asyncio_slow/test_hello_world_callback.py

@@ -0,0 +1,11 @@
+# https://docs.python.org/3.4/library/asyncio-eventloop.html#example-hello-world-callback
+#import asyncio
+import asyncio_slow as asyncio
+
+def print_and_repeat(loop):
+    print('Hello World')
+    loop.call_later(2, print_and_repeat, loop)
+
+loop = asyncio.get_event_loop()
+loop.call_soon(print_and_repeat, loop)
+loop.run_forever()