Add plot.line in Series

[HyukjinKwon]

This PR add series.plot.pie in Series.

Can be tested as below:

import databricks.koalas as ks
import pandas as pd

pdf = pd.DataFrame({
          'a': [1, 2, 3, 4, 5, 6, 7, 8, 9, 15, 50],
      }, index=[0, 1, 3, 5, 6, 8, 9, 9, 9, 10, 10])

kdf = ks.DataFrame(pdf)
pdf['a'].plot.line(colormap='Paired').figure.savefig("image1.png")
kdf['a'].plot.line(colormap='Paired').figure.savefig("image2.png")

In case of this plot, we sample and match the row numbers around 1000.

import databricks.koalas as ks
import pandas as pd

pdf = pd.DataFrame({'a': [1, 2, 3, 4, 5, 6, 7, 8, 9, 15, 50, 100] * 100})
kdf = ks.DataFrame(pdf)
kdf['a'].plot.line(colormap='Paired').figure.savefig("image4.png")

Partially addresses #665

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[HyukjinKwon]

I am merging this for today's release but please feel free to review after it's merged.