Torf

- Fix "Too many open files" error when creating a torrent with loads of small
files.

- Try to reduce memory usage before being out of memory.
- Terminate piece hashing threads if they are idle for more than
500 milliseconds.

- Bugfix: Torrent's piece_size_min and piece_size_max arguments are no longer
ignored.
- Torrent.piece_size_min and Torrent.piece_size_max are now properties. They
default to the new class attributes piece_size_min_default and
piece_size_max_default.
- Torrent.calculate_piece_size(): New arguments: min_size, max_size specify
the minimum and maximum piece size.

- Bugfix: Don't crash if "creation date" is something weird like an empty
string.

- Torrent now accepts piece_size_min and piece_size_max arguments.
- Bugfix: Don't reuse a torrent that has a smaller piece_size than
piece_size_min.

- Bugfix: Fix --max-piece-size