ENH: add cumulative to kdeplot

Closes #91

statsmodels backend only right now. I'll take a look at how much work it would be to copy how they calculate the cdf from the kde.

One aesthetic thing. What do you think of the gap between the right side of the cdf and the right side of the grid?

```python
In [1]: np.random.seed(0)

In [2]: df = pd.DataFrame(np.random.randn(100, 2))

In [3]: import seaborn as sns

In [4]: sns.kdeplot(df[0], shade=True, cumulative=True)
Out[4]: <matplotlib.axes._subplots.AxesSubplot at 0x10d86b090>
```

![sns_cummulative](https://f.cloud.github.com/assets/1312546/2040884/4a4c74fe-89be-11e3-9cad-7f5592f06387.png)

EDIT: here the statsmodels cdf code: https://github.com/statsmodels/statsmodels/blob/master/statsmodels/nonparametric/kde.py#L161