DEP: Make nansum return 0 for all-NaN or empty axis.

Make this happen and remove test parts dependent on numpy version
< 1.9. Fixes test failures in numpy after 1.8 branch.
Merge pull request #3636 from charris/fix-nanfunction-deprecations

DEP: Make nansum return 0 for all-NaN or empty axis.
ENH: Add an "axis" kwarg to `numpy.unique`