publish: Polish/refactor the publishing workflows. * Add a `--preview` option. * The `--list` option gives a nicer output, and prints warnings/errors for incorrect configuration. * Moved most of the `shell` code into a base class that's reusable. * Simplified the code to log publishing to a file. * Nicer output overall, with times.

cov-core==1.15.0
coverage==3.7.1
mock==1.0.1
pytest==2.7.0
pytest-cov==1.8.1
pytest-mock==0.4.3