PEP-8
Use textwrap.dedent() in tests for readability
Tell flake8 to look inside doctests
Bugfix: appending an entry didn't update total work done

Fixes #28
Make 'pip install -e .' work
Make entry addition fast again

Fixes a regression in #21 that occurred after the fix for #28
Fix Python 3 compatibility

Related to #21 and #28.
Mention the bugfix in the changelog
Print GTK+ theme name if --debug is enabled

Should help debug issues like #29 because people often don't know what
the theme is called.
Invert the icon contrast selection logic

Because it was wrong: it picked a bright icon for bright backgrounds and
a dark icon for dark backgrounds.

Add manual overrides because the corrected logic gets it wrong for the
Adwaita, Ambiance, Radiance.

Fixes #29.
Drop the unused prefer_old_tray_icon setting
Disable tray icon by default for new users

I've decided that GNOME 3 is the primary platform (but I will keep
supporting other desktop environments), and the tray icon is not
convenient/necessary under GNOME Shell.

Besides the best contrasting icon selection logic is rather iffy.