Cleanup e.is_Add branch in limit()

This should fix a number issues, when gruntz() is right, but limit() is wrong due to heuristics, like mentioned in [this thread](https://groups.google.com/forum/?fromgroups=#!topic/sympy/61pRw1PINUA) or issues 2641 and 3461.  See also PR #1980.

The price is altered test with Sum(), a second commit.  Now it raises NotImplementedError.