Billboard.py

Added
- Partial support for new Billboard.com UI used for some (but not all) charts.
The `image` attribute is always set to `None` for such charts.

Fixed
- Fix `lastPos` again in response to UI change.
- Raise when HTTP request for `charts()` fails.

Fixed
- Stop sending HTTP header that was causing all requests to fail with HTTP 403.

Fixed
- Fix bug in which `lastPos` was set to the position two weeks prior instead of last week's position.

- This changelog file.
- The `charts` function for listing all charts (40).
- Validation for dates passed to `ChartData` (40).