Added new `subscriptions` path for Organization.

Added a new `subscriptiond` path for `entities.Organization` to
facilitate tests that need to fetch what subscriptions are available to
a given organization. Also added tests.

Finally, I updated docstrings replacing ``:return:`` entries with the
correct value of ``:returns:``.
:+1: For placing each URL on its own line.
:-1: For the indentation used. Try running both flake8 and pylint over this file.