Create envelope with eventNotification argument: enable API callback

[benoitbryon]

As docusign client software (typically the user interface that uses pydocusign), I want to receive notifications when signatures are updated (typically when document has been signed). At the moment, pydocusign has a "signer return URL", where the signer gets redirected after he accepted or refused a signature. But signers may not proceed the redirection. So a callback from DocuSign service itself would be appreciated. There is DocuSign Connect and "eventNotification" argument in create envelope function.

See https://www.docusign.com/p/RESTAPIGuide/RESTAPIGuide.htm#REST%20API%20References/Send%20an%20Envelope.htm