xs with slices
If I have a DataFrame with a MultiIndex, is there any way to slice on the second level?

If I only want one value, I can use `xs`.

For example:

```
wp = pd.Panel(np.random.randn(2, 4, 5), items=['Item1', 'Item2'],
               minor_axis=pd.date_range('1/1/2000', periods=5),
               major_axis=['A', 'B', 'C', 'D'])
ff= wp.to_frame()
```

```
ff.xs("2000-01-01", level="minor", drop_level=False)
```

But what if I want to get all values after "2000-01-02"?
