Port back to Python 2
Reimplement parallel download; UI changes.

- Parallel parts are downloaded in threads (multiprocessing.dummy)
- Parallel downloads are not a special case (the default is one
  connection).
- Use python-progressbar to display a progress bar with ETA and transfer
  speed. The progress bar works for parallel downloads as well.
- Get rid of the Timer class.

The resume option is disabled for now.