VIS: DataFrame.plot drops datetime data when kind is scatter
This works fine:

``` python
In [13]: df = pd.DataFrame(np.random.randn(300), columns=['a'])
In [14]: df['dtime'] = pd.DatetimeIndex(start='2014-01-01', freq='h', periods=300).time
In [15]: df.plot(x='dtime', y='a')
Out[15]: <matplotlib.axes._subplots.AxesSubplot at 0x118fd17f0>
```

This raises a `KeyError`

``` python
In [17]: df.plot(x='dtime', y='a', kind='scatter')
```

We call `df._get_numeric_data()` which excludes datetimes.
May happen for other kinds too.

I'll fix this; just have to decide how much refactoring.

Matplotlib is ok with `datetime.time` values, it chokes on `datetime` values.
