ImageSet: added a method for intersection
S.Integers: added _eval_imageset method
imageset._intersect: removed the dependence on variable name
Merge pull request #7587 from hargup/integer_intersect

Added method of intersection for ImageSet with Integer as baseset
Fix for series expansions in products of symbols with assumptions

For example:
http://nbviewer.ipython.org/gist/jrjohansson/bf7862198171b16d0ff9