hasher issues

Happy new year! :) 

I've been testing the nt library, mostly in terms of generating torrent files, and I found some issues with the hasher, which I have now fixed (hopefully). The high-level problem was that hashing multiple files often resulted in torrents with invalid piece info.  

There are notes added to the code diff next to relevant lines iantocristian@19d2cc365323b3bced3df6fa4ac2eba9a00a7b89. If you get the chance to have a look let me know what you think.

Thanks,
<a href="https://github.com/iantocristian">iantocristian</a>