Perform flake8-driven clean up

A handful of changes are being made to reduce the amount of complaining
that flake8 does. The specifics aren't too important, but the general
changes are:

- `__all__` is being standardized to only include a trailing comma when
  there is only one item, with all items being surrounded by
  parentheses. The latter change is needed for `pep257` to process the
  file.

- Lines are being shorted to fit within 79 characters.

- Docstrings are being added.

- Unused imports are being removed.
Merge pull request #40 from PyGotham/clean-up

Perform flake8-driven clean up