gistfile1.py

#!/usr/bin/env python

import sys, urllib

url = urllib.unquote(sys.argv[1])

def plus13(c):
    i = ord(c)
    if i >= ord('a') and i <= ord('z'):
        i += 13
        if i > ord('z'):
            i -= 26

    return chr(i)

str = ""
for c in url:
    str += plus13(c)

print str