Handled EmailReference

This pull request handles the EmailReference pointer message which has been described in #4.
We're only discussing the internal messaging format used for reliably delivering message data, including attachments, to Qaiain over Azure Queues. When Qaiain picks up the message, it's going to convert the attachments back to whichever MIME type they should have, and send the message via its outgoing API (currently, SMTP).

The reason we're even discussing this is to ensure interoperability. Qaiain's ingoing interface is messages delivered via a Queue, but we need to have reasonable confidence that various clients (.NET, Javascript) can correctly generate the messages.