Ensure `pipermail.stringify()` output is ASCII-only

For example, instead of this:

    {"email":"bar@example.com","name":"Olav Junker Kjær"}

…`pipermail.stringify()` now returns:

    {"email":"bar@example.com","name":"Olav Junker Kj\u00E6r"}

…which is more portable.

Closes #4.