FIX: Scalar timedelta NaT results raise
Currently, coercion of scalar results from `float` to `timedelta64[ns]` passes through `int`, which raises when attempting to coerce `NaN` to `NaT`.

To reproduce:

``` python

import pandas as pd
import numpy as np
pd.Series([np.timedelta64('NaT')]).sum()
# TypeError: reduction operation 'sum' not allowed for this dtype
```

needs tests (put in `tests/test_series.py`, after the `timdelta` operations sections. Need to systematically tests these types of ops (both with and w/o `NaT`). Might also be some tests in `tseries/tests/test_timeseries.py/test_timedeltas.py`

e.g. `sum,mean,max,min`, and that raises on things like `var,std`
 Oh, the problem is that test_nanops.py only covers NaN and ignores NaT.  Hrm.  Any bets on how much breakage this change is going to reveal?
 It looks like `np.timedelta64('NaT')` creates an object of dtype `m8` instead of `m8[ns]`, and the `isnull` can't handle that.  Instead of accepting `m8`, it would probably be better to coerce `np.timedelta64('NaT')` to `np.timedelta64('NaT', 'ns')` whenever it's being added to a Pandas object, if that's feasible.
 you don't handle m8 like that
it's an integer array; use tslib.iNaT
 I don't understand what you mean by that.  I will see if I can get back to it sometime over the weekend.
 OK, clearly I need to wait until I have time to really read this and figure out what's going on.
 have a look at `tseries/test_timedeltas.py`. timedeltas are an odd duck. You generally work on an int64 based series, then take a `timedelta64[ns]` view. note that almost all tests are skipped for numpy < 1.7 (as its broken) for timedeltas.
 @ischwabacher status?
 Sorry, hectic time at home.  I will try to make time for this soon.
 closing in favor of #8268 

this is already fixed in master with Timedelta refactor
