Fixed bug in rTorrent downloader when file paths start with '/'

Noticed that files from rTorrent sometimes start with ```/```, this breaks ```os.path.join``` as it is interpreted as an absolute path when it's actually relative. This issue leads to the renamer failing to find the files, so never processes the movie.

This should fix this issue by stripping the starting ```/``` if the path is truly relative (doesn't start with ```torrent.directory```).