Remove e.is_Add branch in limit():

    This should fix issues, like mentioned in
    https://groups.google.com/forum/?fromgroups=#!topic/sympy/61pRw1PINUA
    or issues 2641 and 3461.
test for limit(Sum(1/x, (x, 1, y)) - 1/y, y, oo), now raises NotImplementedError
Add XFAIL test for issue 3744
Tests for issues 3461 and 2641
Merge pull request #2000 from skirpichev/limit-add-bug

Cleanup e.is_Add branch in limit()