Version 2.0.0

- Sunset now switches your background (or calls your callbacks) mid-way
  through Civil Twilight time.
- *Note:* Sunset 2.0.0 breaks compatibility with previous versions of
  Sunset for users of the `sunset_callback()` callback.
  'sunset_daytime_callback()' and 'sunset_nighttime_callback()' are
  unaffected.