Support numpy 1.10 (master) by adding support for np.cbrt [closes #2936]

Add support for `np.cbrt` (cube root) to `Quantity`, so that astropy can work with numpy master (closing #2936).

@astrofrog: this passes with the numpy development version on my machine, but to test this on travis would need the PR you thought you could do to my branch for #2573. But then I guess we would have to remove it again before merging, since otherwise travis will always test against numpy master, which may not be what we want! Not quite sure how to handle this elegantly...