REGR: fixing Timestamp/Series subtraction, resolves #6648
closes #6648.

Admittedly the conditional in `__sub__` is a little gross now.

I didn't add release notes as this is really just patching up a yet-to-be released item, but please let me know if I should.

Also, the test only runs on numpy >= 1.7, is that okay?

this looks fine.

can you do a quick perf check?
 Perf test is clean.
 thanks!
