BUG: fix Series.plot label setting
Closes https://github.com/pydata/pandas/issues/10119, a regression in 0.16.1

Basically `Series.plot(label='foo')` isn't actually setting the label.

I want to look at one more thing. With this fix you'd need

``` python
s.plot(label='foo', legend=True
```

to get the legend to actually show up. I can't remember what the old behavior was. At least for seaborn, setting a label implies that the legend should be drawn (e.g. `sns.kdeplot(s, label='foo')`)
