Created signal is not triggered.
:+1: 

Signal in playhouse/signal.py:

``` python
def save(self, *args, **kwargs):
        created = not bool(self.get_id())
        pre_save.send(self, created=created)
        super(Model, self).save(*args, **kwargs)
        post_save.send(self, created=created)
```

If I create a model via

``` python
model = Model()
model.id = uuid.uuid4()
model.save(force_insert=True)
```

then created is always `False`, which means that I can never trigger created event.
