Fleshed out email properties

Added things like Subject and Body.
Beginnings of sending email

Doesn't all work right, but at least sends a plain email to specified
people.

Introduced map/flatMap to a couple of types, to make them easier to use.
They aren't fully monadic yet, but might get there.
Email now basically working

Have sent the initial test email successfully.  Subject and Body of the
email are processed before sending, in the context of the recipient.
(So each copy is customized.)