[Email-SIG] fixing the current email module

[Bill Janssen]

I should point out that I also store lots of metadata in the registered
MIME format text/rfc822-headers (defined in RFC 1892), data that doesn't
necessarily conform to the specific set of headers mentioned in RFC822.
It would be nice if the header support in the email package would also
support reading and writing that format.

And MIME multipart is sometimes used in applications other than email.
It would be nice if the MIME parsing part of the email module could be
used for those purposes, as well -- basically without some of the
headers defined in 2822 and 2821.

I think of those two as lower-level standalone libraries used by the
higher-level email library.

Bill