Listlab

although branch is named "listlab" this is both "listlab" and "rot13" assignment
I get the following error on this line when I execute the script on my machine
```
➜  sea-f2-python-sept14 git:(master) ✗ python foo.py
What would you like to ROT13?abc
Traceback (most recent call last):
  File "foo.py", line 18, in <module>
    assert(user_input == rot13(rot13_simple(user_input)))
  File "foo.py", line 9, in rot13_simple
    str[i] = chr(ord(base) + (ord(str[i]) - ord(base) + 13) % 26)
TypeError: 'str' object does not support item assignment
```