Update polar area chart to allow variable arc widths 

I've extended the polarArea chart to allow variable arc widths via a new "fraction" property on each data entry.   If the property is missing, the chart will fall back to the default equal arc polarArea chart.

![image](https://f.cloud.github.com/assets/1063975/1795173/b3e161e4-69ee-11e3-97cc-ee47cde9943f.png)