[#1109] Load torrents with special characters in autoadd dir

self.config["autoadd_location"] is a unicode string, but using
os.listdir(self.config["autoadd_location"]) will return a list of byte string

when some torrents contain non-ascii filename,it may raise UnicodeDecodeError
with os.pats.path.join(),to avoid messing around with encoding, directly using
byte string for file access