* Tulip issue #184: Log subprocess events in debug mode

  - Log stdin, stdout and stderr transports and protocols
  - Log process identifier (pid)
  - Log connection of pipes
  - Log process exit
  - Log Process.communicate() tasks: feed stdin, read stdout and stderr
  - Add __repr__() method to many classes related to subprocesses


* Add BaseSubprocessTransport._pid attribute. Store the pid so it is still
  accessible after the process exited. It's more convinient for debug.

* create_connection(): add the socket in the "connected to" debug log

* Clean up some docstrings and comments. Remove unused unimplemented
  _read_from_self().