Fix packaging issue: subpackages missing from pypi package

[shevron]

This should resolve #56

[ANaumann85]

Now I get an error with a missing argument:

Traceback (most recent call last):
  File "${HOME}/.local/lib/python3.7/site-packages/giftless/wsgi_entrypoint.py", line 7, in <module>
    app = init_app()
  File "${HOME}.local/lib/python3.7/site-packages/giftless/app.py", line 43, in init_app
    transfer.init_flask_app(app)
  File "${HOME}/.local/lib/python3.7/site-packages/giftless/transfer/__init__.py", line 71, in init_flask_app
    adapter.register_views(app)
  File "${HOME}/.local/lib/python3.7/site-packages/giftless/transfer/basic_streaming.py", line 158, in register_views
    ObjectsView.register(app, init_argument=self.storage)
  File "${HOME}/.local/lib/python3.7/site-packages/giftless/view.py", line 30, in register
    return super().register(*args, **kwargs)
  File "${HOME}/.local/lib/python3.7/site-packages/flask_classful.py", line 138, in register
    proxy = cls.make_proxy_method(name)
  File "${HOME}/.local/lib/python3.7/site-packages/flask_classful.py", line 230, in make_proxy_method
    i = cls()
TypeError: __init__() missing 1 required positional argument: 'storage'
unable to load app 0 (mountpoint='') (callable not found or import error)

The last call comes from flask_classfull, where I have version 0.14.2, if that is important.

[shevron]

@ANaumann85 this is not a regression due to this PR (most likely) but a separate issue. I've created a new issue for it (#58) and will update there.