Regression in DataFrameGroupBy.head()
For [this SO question](http://stackoverflow.com/questions/22691010/how-to-print-a-groupby-object) I isolated a regression in `DataFrameGroupBy.head()` between 0.12.0 and 0.13.0.

```
In [1]: df = pd.DataFrame({'A': ['one', 'one', 'two', 'three', 'three', 'one'], 'B': range(6)})
```

In 0.12.0:

```
In [2]: df.groupby('A').head()
Out[2]: 
             A  B
A                
one   0    one  0
      1    one  1
      5    one  5
three 3  three  3
      4  three  4
two   2    two  2
```

which effectively returns the rows of the underlying `.obj` sorted by group. In 0.13.0:

```
In [2]: df.groupby('A').head()
Out[2]: 
             A  B
A                
one   0    one  0
      1    one  1
two   2    two  2
three 3  three  3
      4  three  4
one   5    one  5

[6 rows x 2 columns]
```

which returns the rows of the underlying `.obj` in their original order. The former is more intuitive.
