add extension mechanism

- type is encoded in key, allowing proper typing in Scala & Java
- test cases for Scala & Java are included to ensure that the API makes
  sense and does not break

akka-actor-tests/src/test/java/akka/actor/JavaExtension.java

@@ -0,0 +1,33 @@
+/**
+ *  Copyright (C) 2009-2011 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.actor;
+
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class JavaExtension {
+  
+  static class TestExtension implements Extension<TestExtension> {
+    private ActorSystemImpl system;
+    public static ExtensionKey<TestExtension> key = new ExtensionKey<TestExtension>() {};
+    
+    public ExtensionKey<TestExtension> init(ActorSystemImpl system) {
+      this.system = system;
+      return key;
+    }
+    
+    public ActorSystemImpl getSystem() {
+      return system;
+    }
+  }
+
+  private ActorSystem system = ActorSystem.create("JavaExtension", ActorSystemSpec.javaconfig());
+  
+  @Test
+  public void mustBeAccessible() {
+    final ActorSystemImpl s = system.extension(TestExtension.key).getSystem();
+    assertSame(s, system);
+  }
+  
+}