Double parent slug fix

If you create a page instance and set its parent before saving, it'll end up with a double parent slug.

```python
parent = RichTextPage.objects.create(title="parent")
child = RichTextPage(title="child")
child.set_parent(parent)
print child.slug  # u'parent/parent/child'
```

 ``Page.set_parent`` first stores nothing as the old parent slug, then lets the page generate a slug during ``Page.save`` (already prefixed by the new parent's slug), and then "fixes" the parent slug.