Add a tail_file context manager to the Host class

[JacobCallahan]

This is a pretty common request and should be useful in a number of scenarios. Usage is pretty simple:

    with my_host.session.tail_file("/var/log/messages") as res:
        # do something that creates new messages
    print(res.stdout)

[JacobCallahan]

I was debating whether this should be on the host object itself or on the host's session object. Let me know what you think. The alternative would look like this

    with my_host.session.tail_file("/var/log/messages") as res:
        # do something that creates new messages
    print(res.stdout)

[JacobCallahan]

@rplevka the new version is in now. The downside I forgot about is that it needed to be duplicated for the ContainerSession, but that's not a big deal.

[Griffin-Sullivan]

============== 2 passed in 457.36s (0:07:37) =============

Tests are passing. Very excited to have this in for testing! One minor gripe is that the broker_settings.yaml file is hard coded in the tests, so I had to copy it to my local dir. We should consider changing the tests to look for settings in ~/.broker

[JacobCallahan]

@Griffin-Sullivan great idea! Is that something you're wanting to do or would you prefer I handle that in this PR?

[Griffin-Sullivan]

I can do it in a separate PR.