Write an IPv6 implementation and deployment roadmap draft

ln5 and I started brainstorming about an IPv6 implementation and deployment roadmap. This roadmap should describe how we're planning to implement IPv6 in Tor in multiple steps, and how we're going to deploy it in the Tor network. The roadmap should also contain the various related products in the Tor ecosystem that we need to make IPv6-ready.

The next steps are to get Nick to review our preliminary roadmap draft and then send it to tor-dev for further discussion.

The goal will be to have a draft ready by December 31, 2011 that will be one of the sponsor G deliverables.