add Dataset.listdir

list files matching dataset filename pattern
don't fail to unlink if open() fails
Hefty Downloader refactor

  - use tempfile.mkdtemp instead of datasets/download
  - split open bits into Dataset.__init__ and Dataset.open
  - mark some attributes as "_private"
  - refactor download into add_files, run_workers, join_all
  - delete files if the download fails
revise log levels and messages
Remove unused constant
Add Dataset Downloading Daemon
Add CLI to start daemon or download single statset