Add key to sorting functions
Many python functions  (sorting, max/min) accept a key argument, perhaps they could in pandas too.

.

_The terrible motivating example was this awful hack from [this question](http://stackoverflow.com/a/17157110/1240268).... for which maybe one could do_

```
df.sort_index(key=lambda t: literal_eval(t[1:-1]))
```

_This would still be an awful awful hack, but a slightly less awful one_.
