Merge pull request apache#1291, allow @reference to support custom annotations

Use custom annotations we can rename the annotation as more obvious name and add many default configs.

Author: charpty

dubbo-config/dubbo-config-api/src/main/java/com/alibaba/dubbo/config/annotation/Reference.java

@@ -29,7 +29,7 @@
  */
 @Documented
 @Retention(RetentionPolicy.RUNTIME)
-@Target({ElementType.FIELD, ElementType.METHOD})
+@Target({ElementType.FIELD, ElementType.METHOD, ElementType.ANNOTATION_TYPE})
 public @interface Reference {
 
     Class<?> interfaceClass() default void.class;