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)

@TomAugspurger didn't we have a discussion about not adding this once before? (as its in statsmodels)
 Yep. It's implemented in `seaborn` now. See the very end of [this section](http://www.stanford.edu/~mwaskom/software/seaborn/tutorial/plotting_distributions.html#estimating-the-density-of-the-observations-kdeplot-and-rugplot). It basically wraps statsmodels' / scipy's kde function.
 This was the issue: https://github.com/pydata/pandas/issues/2669

Basically decided it was out of scope for pandas, especially since there are so many great plotting alternatives being built.
 closing as out-of-scope
 Ah, OK. I thought it is kind of `stacked=True` operation though.
