Array support for np.datetime64 and np.timedelta64

This includes support for the basic arithmetic ufuncs.
It also augments ufunc testing with more values (including interesting ones such as 0 or NaN), and fixes a couple buglets found when doing so.

(this is part of issue #499)
from this I get that loop has changed from the plain string it used to be to an object. Maybe document that in the docstring (and my bad, I didn't comment what was the original parameter either).