Add unicode-header handling to send_pgp_mime.py

Add unicode-header handling to send_pgp_mime.py

Also:

Switched
 email.message_from_string()
to
 email.parser.Parser().parsestr()
for parsing the header, for access to the headersonly option.

Adjusted module import order to alphebetize non-mime email modules.

Added return_realname to source_email(), which makes it more useful to
be-handle-mail (currently uncommitted).

Added a doctest for the plain() output and removed redundant
Content-Type line from the doctests (which we'd removed from the
output with the last commit).

Note that many doctests _will_fail_ unless me@big.edu and you@big.edu
are in your gpg keyring.  At some point I should make those addresses
options to --test...