Add Fabric implementation of SSH client

Install screen in the Docker-based SSH server.

Update the SshClient abstract base class to be synchronous and context manager-based; it now has two working implementations.

Add Fabric-based implementation for SSH client.

Add preemptive cleanup, polling on start, and polling on stop to the Dockerized SSH server. It's now much faster without artificial waits.

Progress towards reduced boilerplate in the set of client/server functionality tests. I'd like to go even further to a point like:

@standard
def test_something(client, server):
    client.exec("whoami")

Add remote-only screen + cURL test.

dkmiller / tidbits