Added area diagram functionality

Hello

I like JBchartView, but I missed the possibility to fill the area under the lines to create a area diagram so I added it. I also added the area diagram to the sample.

I tried to follow the code style of the rest of the porject.

![screenshot 2014 04 22 16 58 09](https://cloud.githubusercontent.com/assets/7364108/2766414/8a055d12-ca2e-11e3-9862-102a244af00d.png)