Series specific styles

Hi,

I've modified your library to allow background painting & data point styling on a per series basis.

So you can have different background colours for each series - something that I needed for my project. Also some series can have data points while others do not.

![image](https://cloud.githubusercontent.com/assets/1418689/3813771/0a688518-1cb6-11e4-9a67-2901c01b9a17.png)


Feel free to include them in your version if you would like.

The code is very simple (basic copy n paste) from one class to the other, and I didn't put in javadoc or comments to remain consistent with your style.

Regards,
Richard Le Mesurier