Resampling with NaT in TimedeltaIndex raises MemoryError
`DatetimeIndex` can handle `NaT`:

```
In [2]: dti = pd.DatetimeIndex(['1970-01-01 00:00:00', pd.NaT, '1970-01-01 00:00:02'])

In [3]: pd.DataFrame([2, 3, 5], dti).resample('1s').mean()
Out[3]: 
                       0
1970-01-01 00:00:00  2.0
1970-01-01 00:00:01  NaN
1970-01-01 00:00:02  5.0
```

But `TimedeltaIndex` can't:

```
In [4]: tdi = pd.to_timedelta(['0s', pd.NaT, '2s'])

In [5]: pd.DataFrame([2, 3, 5], tdi).resample('1s').mean()
...
MemoryError:
```

Versions:

```
python: 2.7.11.final.0
OS: Linux
machine: x86_64
pandas: 0.18.1+38.gc7e6f5f.dirty
numpy: 1.10.2
```
