BUG: pd.to_timedelta handles missing data
closes #5437 

**Updated**: Mostly following @jreback's suggestion, but we need `np.timedelta64('NaT')`, not `pd.tslib.iNaT`, because numpy cannot cast `np.datetime64('NaT')` as `np.timedelta64` type, and throws an `AssertionError`.

```
In [1]: pd.to_timedelta(Series(['00:00:01', '00:00:02']))
Out[1]: 
0   00:00:01
1   00:00:02
dtype: timedelta64[ns]

In [2]: pd.to_timedelta(Series(['00:00:01', np.nan]))
Out[2]: 
0   00:00:01
1        NaT
dtype: timedelta64[ns]

In [3]: pd.to_timedelta(Series(['00:00:01', pd.NaT]))
Out[3]: 
0   00:00:01
1        NaT
dtype: timedelta64[ns]
```
