Match absolute tolerance of `assert_allclose` with `allclose` and `isclo...

Note that `rtol` wasn't altered to reduce backward incompatibility. Using an absolute tolerance of 0 prevents many common cases from passing. Also, matching the absolute tolerance of `allclose` and `isclose` makes debugging much easier.

See discussion:

http://mail.scipy.org/pipermail/numpy-discussion/2014-July/070602.html