BUG/VIS: groupby.hist/plot() should pass group keys as labels
At least wherever possible.

``` python
In [2]: df = pd.DataFrame(np.random.randn(30, 2), columns=['A', 'B'])

In [3]: df['C'] = 15 * ['a'] + 15 * ['b']

In [4]: ax = df.groupby('C')['A'].hist()
```

![hist_leg](https://f.cloud.github.com/assets/1312546/2095613/1ba60d22-8eeb-11e3-8672-f2ad14599817.png)
Ideally the group keys would be the legend for the plot.
I can take this one.
