add JavaTestKit, see #1952

- it’s a completely new-written thing in pure Java, so that “protected”
  modifiers actually work and no ghost errors appear wrt. inheriting
  from PartialFunction or similar
- it also features integration with the EventFilter
- all closure-based constructs are modeled as inner classes of the
  JavaTestKit, where the user needs to override a single method which
  will then be executed

akka-docs/scala/code/docs/testkit/TestkitDocSpec.scala

@@ -275,4 +275,15 @@ class TestkitDocSpec extends AkkaSpec with DefaultTimeout with ImplicitSender {
     //#test-kit-base
   }
 
+  "demonstrate within() nesting" in {
+    intercept[AssertionError] {
+      //#test-within-probe
+      val probe = TestProbe()
+      within(1 second) {
+        probe.expectMsg("hello")
+      }
+      //#test-within-probe
+    }
+  }
+
 }