"Nice to have" termination related changes.

[Yawning]

Per pending changes to the pt spec (Stuff that obfsproxy will get once a developer gets around to it), there are a few "nice to have" things that obfsclient could do.

• Graceful shutdown on SIGTERM https://trac.torproject.org/projects/tor/ticket/9732
• Graceful shutdown when stdout is closed https://trac.torproject.org/projects/tor/ticket/10047