R-like range() function
In R, if I supply range(dataframe), I get back a vector of the minimum value in all the dataframe and the maximum value in all the dataframe. 

DataFrame's min() and max() functions operate only on single columns. The way to do it currently is

``` python

minimum, maximum = (min(df.min()), max(df.max())
```

It's not a critical limitation, but it is convenient to have at times.
