I have found that a significant minority of torrent metafiles begin with a not-so-magic  "d12:_info_length", rather than the expected "d8:announce". While this is not explicit in the spec, it is valid bencode and such files work fine with Transmission, uTorrent and official BitTorrent client.
- Modified torrent metafile validation to allow metafiles starting with "d12:_info_length"
- DRYed out the TorrentProvider class's downloadResult function, which was duplicating code from the parent function, as well as never using Bricky's modified validation code in the parent's _verify_download function