Provide factoryMethod in ObjectBuilder

[trung]

It'd be handy that ObjectBuilder can generate object based on the given factoryMethod. It's used for singleton.

For example,

package {
   public class MySingletonClass {
       private var _singleton:MySingletonClass = null;
       public MySingletonClass() {
            new SingletonEnforcer();
       }
       public static function instance():MySingletonClass {
            if (_singleton == null) {
                 _singleton = new MySingletonClass();
            }
            return _singleton;
       }
   }
}
class SingletonEnforcer {}

<ObjectBuilder generator="{MySingletonClass}" factoryMethod="instance" />