Refactor PieChart to use the PieChartColor class

See previous commit message for an explanation...
Refactor PieChartGD to use PieChartColor

- The method processColor() was deleted, as it is clearly no longer
  needed.

- The call to imageRectangle(), which drew the chart's border, has also
  been removed. It was only being used for debugging but it might be a
  good idea to add this an optional feature...