add new client fixture

replaces previous notify_api fixture with the age old:
```
    with notify_api.test_request_context():
        with notify_api.test_client() as client:
```

just runs those two context managers in a yield fixture (new pytest 3.0 feature)

tests/conftest.py

@@ -24,7 +24,7 @@ def notify_api(request):
     return app
 
 
-@pytest.yield_fixture(scope='function')
+@pytest.fixture(scope='function')
 def client(notify_api):
     with notify_api.test_request_context(), notify_api.test_client() as client:
         yield client