GitHub Proposal: Final Cut

Just in case you weren't copied or missed in the commits lists, here's
the final cut for the GitHub proposal.

If we're all in agreement on the implementation, we can use that to
start the survey that will help the decision to move to GitHub.

Please, avoid arguments against Git or GitHub in this review. You'll
have the opportunity to do so in the survey.

My aim is to have this settled in no more than two weeks, so we can
start the survey and the process as soon as possible. I want to
announce the move, if we decide, on the 3.9 release documents, so it's
not a surprise on the next release.