Since the only way to get a rietveld review link for all the changes is to post a single complete patch, I'm doing so in this issue.  Feel free to review based on either this or the separate patches posted in issue 18785, issue 18860, and issue 18890.

There are a couple of bug fixes in this code, but for the most part it is the addition of a new sub-modulej, and a new subclass to the message.py code, plus the accompanying documentation.

Note that this pretty much completes the proposed API additions to the email module.  There may be some additional small tweaks, and there will certainly be some additional support code for specific message header and message types later, but checking this in will mark the completion of the email6 implementation from my point of view.  (The project won't be complete, of course, until the code becomes non-provisional, hopefully in 3.5.)