Session02

Jesse Klein's Ackermann's function module
Series module added as well
Per [PEP 8](https://www.python.org/dev/peps/pep-0008#maximum-line-length),  line length should be limited to 79 characters. Consider using intermediate symbols, such as:

    left = sum_series(n - 2, optional1, optional2)
    right = sum_series(n - 1, optional1, optional2)
    return left + right

or take advantage of "Python's implied line continuation inside parentheses":

    return (sum_series(n - 2, optional1, optional2) +
               sum_series(n - 1, optional1, optional2))