ENH: Special type for qcut/cut to enable sorting.
I recall discussing this before but came up on the mailing list again. Sorting of cut output is always wrong / ugly. I propose creating a str-like that compares with others of the same type as if they were tuples (possibly with nice logic around ranges) - would be facilitated by intro of CategoricalBlock.

Similar to replacing qcut output with this:

``` python
def convert_bins_to_tuple(iterable):
    return [(left, right) for elem in iterable for left, right in elem.strip('()[]').split(',')]
```
