Implement a TransportHelper to connect via a SOCKS-based pluggable transport
This branch adds a TransportHelper
that knows how to encode per-connection parameters in a socks request, and connect via a given pluggable transport to make that request.
(I've decided to defer general proxy support for non-PT channels till after the must-have features for %Arti 1.1.0: Anticensorship ready are done. See #607.)