Write a proposal to add basic pluggable transport support to Tor

We have proposal 180 which describes a way to decouple protocol-level obfuscation from the core Tor protocol in order to better resist client-bridge censorship. The pluggable transport support doesn't need to be feature-complete (whatever that means), but should be sufficient to support basic proxies and transports.

This document will be part of the December 31, 2011 milestone for sponsor G.