ENH/VIS: KDE plot can be cumulative
I sometimes want cumulative density plot for distribution comparison. Added `cumulative` to KDE plot, like `matplotlib.pyplot.hist`.

```
import pandas as pd
import numpy as np

s = pd.Series(np.random.randn(100))
s.plot(kind='kde', cumulative=True)
```

![figure_1](https://cloud.githubusercontent.com/assets/1696302/2985394/e88ddcd2-dc3b-11e3-9f77-381c5cb981ec.png)
