Radar new feature: fillColor depending on the area size

In picture: 

![chart1](https://f.cloud.github.com/assets/841776/748852/5644dca6-e496-11e2-95e9-89f5567173cb.png)
![chart3](https://f.cloud.github.com/assets/841776/748853/62980244-e496-11e2-8626-3ea2ac1966e8.png)
![chart2](https://f.cloud.github.com/assets/841776/748854/63f39b4e-e496-11e2-8c9d-d30330b15060.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]
            }, {
              fillColorProgressive: true,
              fillColorMin: 'rgba(202, 68, 68, 0.7)',
              fillColorMax: 'rgba(68, 202, 68, 0.7)',
              strokeColor: 'rgba(220,220,220,1)',
              pointColor: 'rgba(220,220,220,1)',
              pointStrokeColor: '#fff',
              data: [55, 33, 44, 88, 11]
            }
          ]
        };