added effect to java api

akka-actor/src/main/scala/akka/japi/JavaAPI.scala

@@ -20,6 +20,13 @@ trait SideEffect {
   def apply: Unit
 }
 
+/**
+ * An executable piece of code that takes no parameters and doesn't return any value.
+ */
+trait Effect {
+  def apply: Unit
+}
+
 /**
  + * A constructor/factory, takes no parameters but creates a new value of type T every call
  + */