Add cdfplot
WIP for now. Closes #2669

I'm using statsmodels' KDE implementation right now; it has a `cdf` method, but scipy's gaussian_kde doesn't. I need to check the math, but I think something like doing a cumsum on the density and normalizing by the sum should be the same.

I'm also going to add kwargs for things like the inverse cdf. 

![cdf](https://f.cloud.github.com/assets/1312546/1749251/0287778c-6505-11e3-9dae-cc9e9c37a1de.png)
