Add series.py
Add sum_series function to series.py
Add __name__ == __main__ block to series.py to test functions in file.
fix typo: def __name... to if __name__... in series.py
Fix errors in sum_series function (pass a and b parameters in recursive function).  Fix errors in if __name__... so that correct tests are performed and run.
add shebang line, so that correct version of python is used to run this script.
Add tests with n values less than or equal to 0.