-----------------
Features around settings.
- Feature 34 - ``DocuSignBackend`` uses ``settings.DOCUSIGN_*`` to instanciate
`DocuSign` client. Available settings are ``DOCUSIGN_ROOT_URL``,
``DOCUSIGN_USERNAME``, ``DOCUSIGN_PASSWORD``, ``DOCUSIGN_INTEGRATOR_KEY``,
``DOCUSIGN_ACCOUNT_ID``, ``DOCUSIGN_APP_TOKEN``, ``DOCUSIGN_TIMEOUT``.
- Bug 30 - Handle DocuSign callbacks that have no "decline reason" attribute
(was raising errors in such a situation).