Fixed bug that caused exception for certain number factorization.  Changed error handling from raising exceptions to returning nil.  Removed redundant method.  Updated unit tests to reflect changes.  Streamlined command line usage. Completed comments for documentation.
Bug Fix; Comments: RDoc

Fixed bug that caused exception for certain number factorization.
Changed error handling from raising exceptions to returning nil.
Removed redundant method.  Updated unit tests to reflect changes.
Streamlined command line usage. Completed comments for documentation.