VIS: legend ('None') also added when no label is given
Compare stable http://pandas.pydata.org/pandas-docs/stable/visualization.html#basic-plotting-plot vs dev docs http://pandas-docs.github.io/pandas-docs-travis/visualization.html#basic-plotting-plot

For the simple example of Series plotting, a legens is added with entry None when there should be no legend:

```
ts = Series(randn(1000), index=date_range('1/1/2000', periods=1000))
ts = ts.cumsum()
ts.plot()
```
