rewrite tutorial docs and example

examples/tutorial/tests/test_factory.py

@@ -0,0 +1,12 @@
+from flaskr import create_app
+
+
+def test_config():
+    """Test create_app without passing test config."""
+    assert not create_app().testing
+    assert create_app({'TESTING': True}).testing
+
+
+def test_hello(client):
+    response = client.get('/hello')
+    assert response.data == b'Hello, World!'