v6.0.0

Breaking Changes:

1. remove PRIORITY, RATIO_LIMIT RatioLimit and IdleLimit, use enum Priority, RatioLimitMode and IdleMode instead.
2. File.priority are now enum Priority instead of string.
3. Torrent.priority are now enum Priority instead of string.
4. Torrent.seed_idle_mode are now enum IdleMode instead of string.
5. Torrent.seed_ratio_mode are now enum RatioLimitMode instead of string.