LoadinManager: Adding onStart.

src/loaders/LoadingManager.js

@@ -8,6 +8,7 @@ THREE.LoadingManager = function ( onLoad, onProgress, onError ) {
 
 	var isLoading = false, itemsLoaded = 0, itemsTotal = 0;
 
+	this.onStart = undefined;
 	this.onLoad = onLoad;
 	this.onProgress = onProgress;
 	this.onError = onError;