Radar new feature: fillColor depending on the area size

In picture: 

![chart1](https://f.cloud.github.com/assets/841776/749333/bc48c49c-e4a8-11e2-9a1e-0ec38a4e6840.png)
![chart2](https://f.cloud.github.com/assets/841776/749335/bebe3324-e4a8-11e2-8499-1ba8ad53b6c3.png)
![chart3](https://f.cloud.github.com/assets/841776/749337/c07e5626-e4a8-11e2-998d-b32f1a73bc76.png)

Sample data (first dataset is used to force the bounds, second one is using the feature):

        var data = {
          labels: ['FWs', '3s', 'GIR', 'Ptts', 'PA'],
          datasets: [{
              fillColor: 'rgba(220,220,220,0)',
              strokeColor: 'rgba(220,220,220,0)',
              pointColor: 'rgba(220,220,220,0)',
              pointStrokeColor: '#fff',
              data: [0, 0, 0, 0, 100]
            }, {
              colorProgressive: true,
              colorMin: 'rgba(255, 0, 0, 0.5)',
              colorMax: 'rgba(0, 255, 0, 0.5)',
              pointStrokeColor: '#fff',
              data: []
            }
          ]
        };