Use celery to load data from Wikipedia

quick and dirty
I already set `CELERY_ALWAYS_EAGER`

```python

def test_loader(fx_session):
    with mock.patch('dev.CELERY_ALWAYS_EAGER', True, create=True):
        dbpedia.load_page(1)
        num = fx_session.query(WorkAuthor).count()
        assert num == 100
```