queryset versions of following and followers

Within the Django ecosystem, it is very useful to deal with querysets for things.  It allows for easy integration with other Django apps.  For instance, I am prompted to contribute here because I'm trying to integrate with djangorestframework and drf-extensions which can do nice things automatically....if you give them a queryset

While I would have preferred that the original ```actrstream.models.followers``` and ```actstream.models.following``` returned querysets and versions with "_list" returned lists, to maintain backwards compatibility, I've instead created the following to return querysets:
* ```FollowManager.followers_qs()```
* ```FollowManager.following_qs()```