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
Fix github's issue #2725
Add mpmath's translation for Abs
Merge pull request #2726 from skirpichev/gh2725

Fix github's issue #2725
Merge pull request #2707 from hacman/factorial

remove practice of evaluating factorial as 0 for negative numbers
First commit, created new files
Removed Dyadic, changed names