Series.plot() ignores label keyword
Using pandas 0.16.1, tested in several environments:

``` python
>>> import pandas as pd
>>> s = pd.Series([0, 1])
>>> ax = s.plot(label='stuff')
>>> assert ax.lines[0].get_label() == 'stuff'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError
```

This is a regression since 0.16.0. I think the problem is in this line in pandas.tools.plotting.MPLPlot._compute_plot_data():
`label = self.kwds.pop('label', None)`
It should read:
`label = self.label`
because the label has been popped before and stored in an attribute.

I want to make a pull request but it will take some time to install git, set up a development environment and get my fingers to relearn their way through it all so it will be a lot of opportunities to beat me.
