Massive cleanup/code reorg.

 * Changed obfs4proxy to be more like obfsproxy in terms of design,
   including being an easy framework for developing new TCP/IP style
   pluggable transports.
 * Added support for also acting as an obfs2/obfs3 client or bridge
   as a transition measure (and because the code itself is trivial).
 * Massively cleaned up the obfs4 and related code to be easier to
   read, and more idiomatic Go-like in style.
 * To ease deployment, obfs4proxy will now autogenerate the node-id,
   curve25519 keypair, and drbg seed if none are specified, and save
   them to a JSON file in the pt_state directory (Fixes Tor bug #12605).