Added Generic Relations to the main Changelog model

By adding a generic relation to the ChangeLog model, we can now fetch the actual model instance of the changed model. We can then, through the admin, grab a history of a specific User, for example, and see all the changes being done.

I am also using on my part, this:
```python
def actual_model_instance(self):
		try:
			return self.object_instance
		except Exception, e:
			return self.model
```

to show a column on the admin of the instances themselves.