ENH: all groupby getitem to return groups based on values
allow GroupBy getitem to access group key values

```
In [8]: df = pd.DataFrame({'A': ['foo', 'bar'] * 3,
                                          'B': np.random.randn(6),
                                          'C': np.random.randint(0, 20, 6)})

# this gets you a new grouper
In [22]: df.groupby(['A'])['B']
Out[22]: <pandas.core.groupby.SeriesGroupBy at 0x3152810>

# get based on a value
In [10]: df.groupby(['A']).get_group('bar')
Out[10]: 
     A         B   C
1  bar -1.303892  19
3  bar  0.916340   8
5  bar -0.387371   7

# could be allowed
In [11]: df.groupby(['A'])['bar']

KeyError: 'bar'
```
