API: value_counts on a Categorical Series should have a CategoricalIndex
Thoughts on this?

``` python
In [8]: import pandas.util.testing as tm

In [9]: s = pd.Series(tm.makeCategoricalIndex(k=100))

In [10]: s.value_counts()
Out[10]:
vcKH    40
vXR9    31
Zn8J    29
dtype: int64

In [11]: s.value_counts().index
Out[11]: Index(['vcKH', 'vXR9', 'Zn8J'], dtype='object')
```

Now that we have `CategoricalIndex` (thanks Jeff), should that type be preserved so that `Out[11]` is a `CategoricalIndex`? My use-case (not shown in this example) is when the original categories are ordered, you get your `value_counts` and then want to sort the index.
