remove unused method TorrentDef.retrieve_from_magnet_infohash()
change some objects to use super()
some small changes
fix for get torrent from magnet link
add debug
update comments
Merge pull request #653 from LipuFei/fix-get-from-magnet

Fix get torrent from magnet problem
Merge pull request #654 from LipuFei/trace-dup-swifttorhash

Trace duplic swift torrent hash problem
Use returnvalue of SwiftDef finalize, log error if roothash could not be
computed
Fixed return of finalize
Merge pull request #656 from NielsZeilemaker/swift_roothash_fix

Use returnvalue of SwiftDef finalize, log error if roothash could not be