API: DatetimeIndex / TimedeltaIndex 2d slicing should result in Index
Following slices basically result in the same `Index`.

```
import pandas as pd
pd.Index([1, 2, 3])[:, None]
# Int64Index([[1], [2], [3]], dtype='int64')
pd.MultiIndex.from_tuples([('a', 1), ('b', 2)])[:, None]
# MultiIndex(levels=[[u'a', u'b'], [1, 2]],
#            labels=[[[0], [1]], [[0], [1]]])

pd.period_range('2011-01-01', freq='M', periods=3)[:, None]
# PeriodIndex(['2011-01', '2011-02', '2011-03'], dtype='int64', freq='M')
```

But `DatetimeIndex` and `TimedeltaIndex` doesn't because of following line.
- https://github.com/pydata/pandas/blob/master/pandas/tseries/base.py#L119

```
pd.date_range('2011-01-01', freq='M', periods=3)[:, None]
# array([['2011-01-31T09:00:00.000000000+0900'],
#        ['2011-02-28T09:00:00.000000000+0900'],
#        ['2011-03-31T09:00:00.000000000+0900']], dtype='datetime64[ns]')
pd.timedelta_range('1 days', freq='D', periods=3)[:, None]
# array([[ 86400000000000],
#        [172800000000000],
#        [259200000000000]], dtype='timedelta64[ns]')
```

I understand these must be `Index` also, or any reason for this?
