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.
Add a test targeting BZ 1151220

Clients must submit a nested hash of attributes to certain API URLs.

For example, POSTing this data to the `/architectures` URL will succeed:

    {'name': 'foo'}

However, these operating system IDs will be silently ignored:

    {'name': 'foo', 'operatingsystem_ids': [1, 2, 3]}

A client can hack around this oddity by wrapping submitted data inside an extra
hash:

    {'architecture': {'name': 'foo', 'operatingsystem_ids': [1, 2, 3]}}

This extra hash should not be necessary. It makes the API harder to use, and
the extra hash is only required for some API calls.
Merge pull request #1541 from Ichimonji10/bz-1151220

Add a test targeting BZ 1151220