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:`` and disabled some pylint directives.
Merge pull request #1536 from omaciel/add-subscriptions-api

Added new `subscriptions` path for Organization.