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Integrate more DocuSign features.
- Feature 1 - Envelopes can be created with ``eventNotification`` argument,
which allows to setup API callbacks on signature updates.
- Feature 3 - Added support for "approve tabs", which allow signer to sign a
document without adding visible text/graphic to the document.
- Feature 5 - DocuSignClient.login_information raises DocuSignException and
emits logs on error.