Extract Envelope, DKIM, SPF, Spam Score, and Spam Report in SG Adaoter

These fields are useful for the following reasons:
 - Envelope: Only way to get the recipient email address in the case of a BCC
 - DKIM, SPF: Useful sender verification metrics from SendGrid
 - spam_score, spam_report: Useful spam metrics from SendGrid
Update README.md

Add DKIM, SPF, Spam Score, Spam Report, and Envelope to README
Merge pull request #1 from ygnessin/extract-misc-sendgrid-fields

Extract misc sendgrid fields