should fix issue #7520, test_nanargmin fails with datetime64 objects
I think this will fix the issue #7520.  The issue appears to be that older versions of `numpy`, probably less than `1.7.0`, can't handle `datetime64` dtypes very well.  I have added simple check to see if `numpy` can handle a standard operation with `datetime64` dtypes, and if not it skips those tests.

I can't test this personally since I don't have the older version of `numpy`.

all datetime64/timesela64 HAVE TO be cast to i8 before operations (and cast back after)

it is an assertion error if this is not the case
 you can simply set up a virtual env to test numpy 1.6.1
 Yes, `nanops` casts internally.  The problem with this bug is with the reference function.  All the `nanops` tests compare to a reference function to make sure the calculations are correct.  In this case, it is a `numpy` function.  The current version of the `numpy` function, and the version that travis tests against, handle `datetime64` objects fine.  Older versions, however, apparently don't.

So this change just tests the `numpy` reference function and if it fails, it skips the test.
 that's fine then
