Transport over Mix

This is intended to become a specification and reference implementation of a transport abstraction layer over the Mix Protocol.

The idea is to hide the packet size and other limitations of Mix behind a nice abstraction layer, so applications can pretend they are communicating over a "normal" network not unlike TCP (a very slow, and moderately reliable, but otherwise pretty normally behaving network socket).

Furthermore, we also take the opportunity to document (including an executable specification) both the Sphinx mix packet format, and SURBs (Single Use Reply Blocks).