asyncio_slow: Add coroutine example from docs.

asyncio_slow/test_hello_world.py

@@ -0,0 +1,12 @@
+#https://docs.python.org/3.4/library/asyncio-task.html#example-hello-world-coroutine
+#import asyncio
+import asyncio_slow as asyncio
+
+@asyncio.coroutine
+def greet_every_two_seconds():
+    while True:
+        print('Hello World')
+        yield from asyncio.sleep(2)
+
+loop = asyncio.get_event_loop()
+loop.run_until_complete(greet_every_two_seconds())