Not able to change direction of sort on per column basis for DataFrame
The `sort` method on DataFrame supports sorting by multiple columns, yet only supports sorting in one direction (ascending or descending). It would be great to specify this. Perhaps something like this: `df.sort(columns=['Name|descending', 'Age']` which would sort name first in descending order and then where names match use ascending order.

Also I see no mention of whether the sort is stable. If it is, this can be replicated by multiple sort calls.
