Remove decoy

This doctest was half-written then abandoned when I decided unittests
were a better approach.  It was never included in the test suite.
Python 2.6 support

- Add argparse to install_requires if necessary
- Add and use unittest2 to get 'with self.assertRaises() as cm:'
- Avoid 'with a, b, c:', use nested with statements
- Reimplement subprocess.check_output inline in the tests

Closes #22