Update src/main/java/br/com/digilabs/jqplot/elements/RendererOptions.jav...

I added fillToZero (Boolean) property.
It's usefull for BarChart with negative values.
Without a fillToZero set to true, the BarChart will render negative values in the same "Orientation" than positive values.

I attached two screenshot (before and after).

![barchar negative value before](https://f.cloud.github.com/assets/2108652/145576/3b50004e-7469-11e2-9ade-552b53c104ed.png)
![barchar negative value after](https://f.cloud.github.com/assets/2108652/145577/40d70ac6-7469-11e2-9a2a-fca8a888e15c.png)