Updates for the new Email Activity API (beta)

[thinkingserious]

SendGrid has added an API to download information from the Email Activity feed. There are four new endpoints:

• GET /messages
• GET /messages/{msg_id}
• POST /messages/download/
• GET /messages/download/{download_uuid}

A getting started doc for the API is here.

The following items need to be updated in this repo:

• USAGE.md
• /examples (create a messages folder with a file messages.py)
• Tests
• USE_CASES.md

[dhoeric]

Hi, I would like to pick up this issue :)