Basic blog structure (fix #3)

Ref: #3.

This also intruce swebtest.
This also adds TimestampedModel in ideasbox models.
Ah, sorry, I didn't understand your answer. The same fixture could be written like that:
```python
@pytest.yield_fixture
def app():
    wtm = django_webtest.WebTestMixin()
    wtm._patch_settings()
    yield django_webtest.DjangoTestApp()
    wtm._unpatch_settings()
```

I find that much cleaner, but there's a few limitations (see the last part of the doc), and it's pretty new (and could evolve/change in the future), so it's really a matter of taste.