Added ModelFactory. This replaces the 'models' dict in the tweepy.models module.

Added ModelFactory. This replaces the 'models' dict in the tweepy.models module.

This will allow for more flexible plug 'n play for developers that need
to extend Tweepy's models. To use custom models, they will extend the ModelFactory
and then pass this new factory into the API constructor.

Example:
    class MyStatus(Status):
        """A extended Status model"""

    class MyModelFactory(ModelFactory):
        status = MyStatus

    api = API(model_factory=MyModelFactory)