Added Library model to have more granular federation management

api/funkwhale_api/federation/actors.py

api/funkwhale_api/federation/factories.py

...hale_api/federation/migrations/0003_auto_20180406_1621.py

api/funkwhale_api/federation/models.py

api/funkwhale_api/music/factories.py

...funkwhale_api/music/migrations/0023_auto_20180406_1621.py

api/funkwhale_api/music/models.py

api/funkwhale_api/music/serializers.py

api/funkwhale_api/music/tasks.py

api/tests/federation/test_actors.py

api/tests/federation/test_models.py

api/tests/federation/test_views.py

api/tests/music/test_import.py

api/tests/music/test_serializers.py