add Class[_] to LogEvent

- it is customary to use class name for categorizing logs, hence we
  should support it; class is taken from logSource.getClass
- update SLF4J module to use logClass as category and set logSource in
  MDC "akkaSource"
- add docs

akka-docs/scala/code/akka/docs/event/LoggingDocSpec.scala

@@ -38,11 +38,11 @@ object LoggingDocSpec {
 
   class MyEventListener extends Actor {
     def receive = {
-      case InitializeLogger(_)              ⇒ sender ! LoggerInitialized
-      case Error(cause, logSource, message) ⇒ // ...
-      case Warning(logSource, message)      ⇒ // ...
-      case Info(logSource, message)         ⇒ // ...
-      case Debug(logSource, message)        ⇒ // ...
+      case InitializeLogger(_)                        ⇒ sender ! LoggerInitialized
+      case Error(cause, logSource, logClass, message) ⇒ // ...
+      case Warning(logSource, logClass, message)      ⇒ // ...
+      case Info(logSource, logClass, message)         ⇒ // ...
+      case Debug(logSource, logClass, message)        ⇒ // ...
     }
   }
   //#my-event-listener