Allow lazy loading of the Box3 constructor, so that webpack can load three js...

Allow lazy loading of the Box3 constructor, so that webpack can load three js source files directly instead of requiring all of the THREE.

src/math/Sphere.js

@@ -28,10 +28,12 @@ Sphere.prototype = {
 
 	setFromPoints: function () {
 
-		var box = new Box3();
+		var box;
 
 		return function setFromPoints( points, optionalCenter ) {
 
+			box = box || new Box3();
+
 			var center = this.center;
 
 			if ( optionalCenter !== undefined ) {