Improve progress bar dramatically; better comments

 - Fix progress bar which was failing because the callback kept
   re-initializing it.

 - Monkeypatch ProgressBar to force update via time-elapsed instead of
   only updating when the whole-integer percentage increments, which is
   terrible for large file transfers

 - Implement Position() as a ProgressBarWidget to track filesize
   completed.

 - Lints clean with flake8