#16 新增createEntityAsync方法

修复entity.id为null

source/src/ECS/Entity.ts

@@ -1,6 +1,6 @@
 module es {
     export class Entity {
-        public static _idGenerator: number;
+        public static _idGenerator: number = 0;
 
         /**
          * 当前实体所属的场景

source/src/ECS/Scene.ts

@@ -262,13 +262,23 @@ module es {
             return this.addEntity(entity);
         }
 
+        /**
+         * 将实体添加到此场景，并返回它
+         * @param name
+         */
+        public createEntityAsync(name: string): Promise<Entity> {
+            return new Promise<Entity>(resolve => {
+                resolve(this.createEntity(name));
+            });
+        }
+
         /**
          * 在场景的实体列表中添加一个实体
          * @param entity
          */
         public addEntity(entity: Entity) {
             if (this.entities.buffer.contains(entity))
-                console.warn(`You are attempting to add the same entity to a scene twice: ${entity}`);
+                console.warn(`您试图将同一实体添加到场景两次: ${entity}`);
             this.entities.add(entity);
             entity.scene = this;