Fix torrent download

With libtorrent versions shipped by recent Ubuntu and Debian versions.
Finish swift cleanup on debian/ started by Lipu.
Do not try to sign the swift.exe binary.
Remove swift from LICENSE.txt
This should have been deleted a long time ago.
Delete obsolete files.
Merge pull request #944 from whirm/fix_debian_packaging

Finish swift cleanup on debian/ started by Lipu.
Merge pull request #945 from whirm/fix_and_clean_packaging

Fix and clean packaging
Merge pull request #943 from whirm/fix_download_torrent

Fix torrent download