BUG: pd.to_timedelta raises instead of returning NaT
```
In [1]: Series(['2011-01-01', np.nan]).apply(pd.to_datetime)
Out[1]: 
0   2011-01-01 00:00:00
1                   NaT
dtype: datetime64[ns]

In [2]: Series(['00:00:01', np.nan]).apply(pd.to_timedelta)
AssertionError: Invalid type for timedelta scalar: <type 'float'>
```

May not be a trivial fix. So far I have seen that inserting np.nan and pd.tslib.iNaT into the result turns the dtype to object, which is no good.
