Added ggplot examples.

I added three ggplot-based examples we can actually render with mpl support.

The plots are the following:

![density](https://cloud.githubusercontent.com/assets/1640669/2623991/9e2ce2f2-bd17-11e3-9946-b063c65a4988.png)

![line](https://cloud.githubusercontent.com/assets/1640669/2623992/a228e7e8-bd17-11e3-8115-10ee5fe11de7.png)

![step](https://cloud.githubusercontent.com/assets/1640669/2623993/a92c6740-bd17-11e3-9415-ef8f3bc7ebac.png)

 
I also reorganized the mpl examples into mpl, ggplot and seaborn folders.