Add missing package subprocess.api.

Missing package caused import error on fresh installs.

- Add test confirming import error.
- Add missing package.

---

Running from the command line after a fresh install caused the following error:

```sh
$ shellstreaming --help
Traceback (most recent call last):
  File "/usr/local/bin/shellstreaming", line 5, in <module>
    pkg_resources.run_script('shellstreaming==0.1.0', 'shellstreaming')
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 489, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/pkg_resources.py", line 1207, in run_script
    execfile(script_filename, namespace, namespace)
  File "/Library/Python/2.7/site-packages/shellstreaming-0.1.0-py2.7.egg/EGG-INFO/scripts/shellstreaming", line 10, in <module>
    from shellstreaming.master.master import main
  File "/Library/Python/2.7/site-packages/shellstreaming-0.1.0-py2.7.egg/shellstreaming/master/master.py", line 32, in <module>
    from shellstreaming import api
ImportError: cannot import name api
```