Fix Issue 3727 (ufuncify + erf)
Merge pull request #1953 from thisch/issue_3727

Fix Issue 3727 (ufuncify + erf)

http://code.google.com/p/sympy/issues/detail?id=3727
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
Try to isolate gruntz bug, comment out test 8.11 (DON'T MERGE!)