Making categoricals comparable
another example:

http://stackoverflow.com/questions/23814368/sorting-pandas-categorical-labels-after-groupby

At the moment bins / buckets are simply strings, so you can't sort them properly.

```
A=pd.qcut(X['x'], [0,0.25,0.5,0.75,1.0])

In [28]: A[0]
Out[28]: '(0.635, 2.4]'

In [29]: type(A[0])
Out[29]: str
```

 The dtype of the Index is object (because these are strings), maybe we could just have these as a subclass of strings where:

```
def __lt__(self, other):
    from ast import literal_eval
    return literal_eval(self) < literal_eval(other)
```

Can put together pr if seems reasonable.
