I want to sort a data frame by multiple columns. Here is a simple data frame I made. How can I sort each column by a different sort type? 

```
using DataFrames

DataFrame(b = ("Hi", "Med", "Hi", "Low"),
      levels = ("Med", "Hi", "Low"),
      x = ("A", "E", "I", "O"), y = (6, 3, 7, 2),
      z = (2, 1, 1, 2))

```

Ported this over from [here](https://stackoverflow.com/questions/1296646/how-to-sort-a-dataframe-by-multiple-columns).