Status model from Model Factory

I was confusing at first when sometimes the models from the factory are called like:
````
status_model = getattr(api.parser.model_factory, 'status') if api else Status
````
Example: [here](https://github.com/tweepy/tweepy/blob/master/tweepy/models.py#L76)
And sometimes in this way:
````
 setattr(user, k, Status.parse(api, v))
````
Example: [here](https://github.com/tweepy/tweepy/blob/master/tweepy/models.py#L123)

I think the best way is always call the models instances from the factory, but like that requires a more refactoring I've created this PR just with the SearchResult model.

Please give your thoughts.