complete paths and file parts of session04 assignment

These are the first two parts of the last session04 assignment, but does not include the "dict" changes to mailroom
intereting -- I woulnd'n hav thought to do it this way!

simplest:
str_rot13 += new_c

most pythonic:
str_list = []
...in loop
    str_list.append(new_c)
return "".join(str_list)