Ubuntu 17.04 provides Docker 1.12.6, which has API version 1.24.
`dev_requirements.txt` specifies the docker-py 2.5.1, which by default
requests API version 1.30.

Hence when the SSH unit tests try to run the container specified in
`DockerizedSshDaemon` an error occurs

```
APIError: 400 Client Error: Bad Request ("client is newer than server
(client API version: 1.30, server API version: 1.24)")
```

I presume that `u1704` is a relic of a bygone container/host.

On Ubuntu 17.10 something (probably Docker) appears to be accepting
connections, before sshd is fully ready. This results in a race
condition, and hence connection errors for the first few tests (2-3 on
my laptop).

testlib.wait_for_port() checks not only that the port can be connected
to, but also something resembling the sshd banner is sent.

Fixes #51

These will help with future PyPI discovery, and should make it clear
that Python 3.x is not (yet) supported.

- Uses /usr/bin/env so it still works in e.g. a virtualenv
- Uses python2, rather than python3 to indicate mitogen currently being
2.x only.

py_packages is not a field know by distutils or setuptools. The closest
is `py_modules`, which perhaps what the erroneus line here started as.

Fixes #43

Without this, it's possible for Waker to be start_received() after the
shutdown signal has already been sent, resulting in 5 second delay
during shutdown.

Additionally mask EBADF during os.write() to waker's write side.
Necessary since nothing synchronizes writer threads from the broker
thread during shutdown. Could be done with a lock instead, but this is
cheaper.

Can't figure out what it's supposed to do any more, and can't find a
version of Ansible before August 2016 (when I wrote that code) that
seems to need it.

Add some more mitigations to avoid sending dylibs.