Stop using Twisted Reactor which is the main event loop of Twisted
Do not print private key to console
Merge pull request #737 from yagoulas/drop-reactor

Drop reactor
Drop all references to twisted.
Drop Twisted from requirements.
Upgrade tornado to version 4.0.2.
Remove logging of settings
Merge pull request #751 from dionyziz/hide_private_key

Do not print private key to console
Merge pull request #753 from Renelvon/bump_tornado

Upgrade tornado to version 4.0.2.
Merge pull request #752 from Renelvon/drop_twisted

Drop twisted from requirements
Refactor code in connection.py

* Drop unused PeerConnection.responses_received.
* Refactor PeerConnection.create_zmq_socket.
  * Allow it to raise an exception; we are catching it elsewhere.
  * Make it a private function.
* Refactor PeerConnection.send_raw.
  * Make failure cases stand out.
  * Do not compress until ready to send.
* Make failure cases stand out in CryptoPeerConnection.start_handshake.
* Refactor and DRY CryptoPeerConnection.check_port.
  * Prefer ipv6 over ipv4 sockets.
  * Remove hardcoded timeout. Use the instance attribute.
* Shorten CryptoPeerConnection.sign.
* Straighten CryptoPeerConnection.encrypt.
* Refactor CryptoPeerConnection.send.
  * Make failure cases stand out.
  * Guard against encrypt returning False.
* Remove PeerListener.cleanup_context. PeerListener no longer manages
  it own zmq.Context. Purge any references.
* Straighten PeerListener.listen.
* PEP8 fixes proposed by gubatron.