Switch back to only one Gradient constructor for both linear and radial Gradients.

examples/JSON/Gradients.html

@@ -7,7 +7,7 @@
 	<script type="text/javascript" src="../../dist/paper.js"></script>
 	<script type="text/paperscript" canvas="canvas1">
 		var path = new Path.Circle(view.center, view.bounds.height * 0.4);
-		var gradient = new RadialGradient('yellow', 'red', 'black');
+		var gradient = new Gradient(['yellow', 'red', 'black'], true);
 		var from = path.position;
 		var to = path.bounds.rightCenter;
 		var gradientColor = new GradientColor(gradient, from, to);