remove practice of evaluating factorial as 0 for negative numbers
it is infinity for negative integers and remains unevaluated for non-integers. fix gosper test, which had a missing term.
fix string representations
doctest fix
use complex infinity for factorial of negative integers
note that the factorial change fixes #2703 on github
Merge pull request #2707 from hacman/factorial

remove practice of evaluating factorial as 0 for negative numbers