How can you tell an extrovert from an
introvert at NSA? Va gur ryringbef,
gur rkgebireg ybbxf ng gur BGURE thl'f fubrf.
I found this joke on USENET, but the punchline is scrambled. Maybe you can decipher it?
According to Wikipedia, ROT13 (http://en.wikipedia.org/wiki/ROT13) is frequently used to obfuscate jokes on USENET.
Hint: For this task you're only supposed to substitue characters. Not spaces, punctuation, numbers etc.
Test examples:
```
rot13("EBG13 rknzcyr.") == "ROT13 example.";
rot13("This is my first ROT13 excercise!" == "Guvf vf zl svefg EBG13 rkprepvfr!"
```
from string import ascii_letters, ascii_lowercase, ascii_uppercase

def shift(s, n):
    return s[n:] + s[:n]

table = str.maketrans(
    ascii_letters,
    shift(ascii_lowercase, 13) + shift(ascii_uppercase, 13))

def rot13(message):
    return message.translate(table)

from string import ascii_letters, ascii_lowercase, ascii_uppercase

def shift(s, n):
    return s[n:] + s[:n]

table = str.maketrans(
    ascii_letters,
    shift(ascii_lowercase, 13) + shift(ascii_uppercase, 13))

def rot13(message):
    return message.translate(table)
