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-remote/src/test/scala/akka/remote/RemoteDeathWatchSpec.scala

@@ -15,8 +15,10 @@ akka {
         deployment {
             /watchers.remote = "akka://other@127.0.0.1:2666"
         }
+        debug.lifecycle = on
     }
     cluster.nodename = buh
+    loglevel = DEBUG
     remote.server {
         hostname = "127.0.0.1"
         port = 2665