Cloning for WidgyPages

Puts a 'Clone' button on the widgy mezzanine WidgyPage admin (next to 'View on site'). Clicking it will ask you for a new title and create a new page with a cloned version of the widgy content, including the
working copy and history.

Cloning of the VersionTracker itself is done through its new `clone` method.
Idea,

instead of this, why not have the commits property return a QuerySet for the ReviewedVersionCommit:

```python
@property
def commits(self):
    return self.commit_model._default_manager.filter(tracker_pk=self.pk)
```

This code could even live in the VersionTracker model.  It would mean changes in other parts of the code, but I feel like those would be improvements.

Thoughts?