Give pandas.core.groupby.generic.DataFrameGroupBy __str__/_repr_pretty_/__repr__ methods
Current behavior:

```
>>> df = pd.DataFrame({'a': [1, 1, 3], 'b': [9, 8, 7]})
>>> df.groupby('a')                                                                                                                       
<pandas.core.groupby.generic.DataFrameGroupBy object at 0x7fd3d5a7b470>
```

Desired behavior (example, not exact):

```
>>> df = pd.DataFrame({'a': [1, 1, 3], 'b': [9, 8, 7]})
>>> df.groupby('a')
group a=1
   a  b
0  1  9
1  1  8

group a=3
   a  b
2  3  7
```