Error when file modified after hash check

[ieee8023]

Because hash checking is limited to not happen that often there is an issue with a file being modified and then the academictorrents not noticing. We could add extra checks that would prompt a hash check. Some ideas are:

1. If the file size on disk does not match the file size of the torrent.
2. If the date modified of the downloaded is after the date of last hash check.
3. If the number of files on disk is less than the number of files in the torrent.