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]