Add a layer to the document's children list when created and support activeLayer.

src/Doc.js

@@ -1,14 +1,19 @@
 Doc = Base.extend({
 	initialize: function(canvas) {
+		if(canvas) {
 			this.canvas = canvas;
 			this.ctx = this.canvas.getContext('2d');
 			this.size = new Size(canvas.offsetWidth, canvas.offsetHeight);
-		this.children = [];
+		}
+		this.activeLayer = new Layer();
+		this.children = this.activeLayer;
 	},
 	redraw: function() {
+		if(this.canvas) {
 			this.ctx.clearRect(0, 0, this.size.width, this.size.height);
 			for(var i = 0, l = this.children.length; i < l; i++) {
 				this.children[i].draw(this.ctx);
 			}
 		}
+	}
 });