Test for Slf4jEventHandler. See #1625

* Test of MDC parameters and other output using a custom logback appender
* Logback test dependency
* Changed to encoding instead of deprecated layout

akka-sbt-plugin/sample/src/main/config/logback.xml

@@ -3,9 +3,9 @@
 <configuration>
 
 <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
-  <layout>
+  <encoder>
     <pattern>%date{ISO8601} %-5level %X{akkaSource} %X{sourceThread} - %msg%n</pattern>
-  </layout>
+  </encoder>
 </appender>
 
 <root level="info">