ENH/VIS: Pass DataFrame column to size argument in df.scatter
You can already kind of do this by passing in the numpy array

```
In [83]: df = pd.DataFrame(np.random.randn(100, 2))

In [84]: df['z'] = np.random.uniform(size=(100))

In [85]: df.plot(kind='scatter', x=0, y=1, s=df.z.values * 1000)
Out[85]: <matplotlib.axes._subplots.AxesSubplot at 0x11a81df60>
```

![p](https://cloud.githubusercontent.com/assets/1312546/4237073/ea994988-39cb-11e4-9fd5-df3ad45ddaba.png)

But when I merge https://github.com/pydata/pandas/pull/7780/  (coloring by column) it would be natural  (and awesome) to do `df.plot(kind='scatter', x='x', y='y', c='color', s='size')`

Shouldn't be too hard if we're willing.
