handle NaT add/sub in one place
ATM DatetimeIndex/TimedeltaIndex/PeriodIndex each have special-casing for addition/subtraction of NaT.  This checks for NaT up-front and puts all of that logic into add_nat and sub_nat methods.

Also makes another step towards centralizing all of the wrapping of the add/sub ops at the end of `__add__` and `__sub__`.

Should be orthogonal to other outstanding PRs, albeit just barely.