convert factorials to gamma-function format
adding factorial test case
be more explicit with the rewrite operation
stripping trailing whitespace
adding negative number test case
only rewrite for positive z0
use is_positive in favor of __ge__

relevant issue: http://code.google.com/p/sympy/issues/detail?id=2832
adding symbolic z0 test case for factorials
Merge pull request #1734 from perimosocordiae/master

Fix for #1733: mishandling of factorials when taking limits