On second thought instead of using log.Panicf(), panic() and do the
logging with recover().  This somewhat centralizes logging in
obfs4proxy, which will be easier to change when I invariably decide to
do logging differently in the future.

This fixes #1.

 * The old and the busted: obfs4-[client,server].
 * The new hotness: obfs4client.
 * Add obfs4.ServerHandshake() that servers need to call after a
   successful return from Accept().  This allows implementations to
   move the handshake into a goroutine or whatever.