Modularize configuration. See #1361

* Split config reference to one for each module/extension.
* Adjusted signature of registerExtension to avoid race of extension init
* Moved Duration.dilated to testkit
* TestKitExtension
* RemoteExtension
* SerializationExtension
* Durable mailboxes extensions
* Fixed broken serialization bindings and added test
* Updated configuration documentation
* System properties akka.remote.hostname akka.remote.port replaced with akka.remote.server.hostname and akka.remote.server.port
* Adjustments of ActorSystem initialization. Still don't like the two-phase constructor/init flow. Very fragile for changes.

Review fixes. SerializationExtension

akka-docs/general/code/ConfigDocSpec.scala

@@ -0,0 +1,32 @@
+package akka.docs.config
+
+import org.scalatest.WordSpec
+import org.scalatest.matchers.MustMatchers
+
+//#imports
+import akka.actor.ActorSystem
+import com.typesafe.config.ConfigFactory
+import com.typesafe.config.ConfigParseOptions
+
+//#imports
+
+class ConfigDocSpec extends WordSpec {
+
+  "programmatically configure ActorSystem" in {
+    //#custom-config
+    val customConf = ConfigFactory.parseString("""
+      akka.actor.deployment {
+        /app/my-service {
+          router = round-robin
+          nr-of-instances = 3
+        }
+      }
+      """, ConfigParseOptions.defaults)
+    val system = ActorSystem("MySystem", ConfigFactory.systemProperties.withFallback(customConf))
+    //#custom-config
+
+    system.stop()
+
+  }
+
+}