The sort option of groupby does not work as stated
In the documentation (http://pandas.sourceforge.net/groupby.html) step 349, the sort option does not work as stated. If one repeats the steps he gets:

```
In [38]: df2 = DataFrame({'X' : ['B', 'B', 'A', 'A'], 'Y' : [1, 2, 3, 4]})

In [39]: df2.groupby(['X'], sort=True).sum()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)

/home/tcebrian/<ipython console> in <module>()

TypeError: groupby() got an unexpected keyword argument 'sort'
```
