Add a CLI helper

[Ichimonji10]

Add, document and test module pulp_smash.cli. This module provides "tools for
working with Pulp's CLI." This fixes #32:

It'd be nice if we could inspect the state of remote systems under test. For
example, it may be useful to log in to a remote system and determine whether
certain files are lying around in certain places in the file system.

Here's a simple example of how to use the new module:

>>> from pulp_smash import cli, config
>>> server_config = config.ServerConfig('localhost')
>>> client = cli.Client(server_config)
>>> response = client.run(('echo', '-n', 'foo'))
>>> response.returncode == 0
True
>>> response.stdout == 'foo'
True
>>> response.stderr == ''
True

The Client class is flexible enough that both request and response handling
can be customized on a per-instance basis. See the docstrings in module
pulp_smash.cli for details.

Add "plumbum" as a dependency. It is used by the CLI helper.

Many thanks to @elyezer for help in brainstorming a solution to this issue.

This commit does not affect test results (107 tests):

============  =============================
Pulp Version  Test Suite Results
============  =============================
2.7           OK
dev (2.8)     FAILED (errors=2, skipped=13)
============  =============================

[Ichimonji10]

@bmbouter

[elyezer]

Breadcrumbs++

[bmbouter]

Does this need any representation in the docs or do the docstrings create the docs? If not, I think the docstring examples can be moved into the docs or have the docs build from them so we don't duplicate them.

[Ichimonji10]

The documentation is built directly from the docstrings.

[elyezer]

The travis failure is ./pulp_smash/cli.py:7:1: F401 'warnings' imported but unused.

[Ichimonji10]

Updated.

[elyezer]

ACK

[bmbouter]

This looks good to me. +1 to merging it.