Makes SubstituteLogger location aware

Currently, `SubstituteLogger` neither supports location information from upstream callers nor passes location information to the delegated logger.

This PR:

- implements `LocationAwareLogger`, which allows upstream callers to pass location information both through the `LocationAwareLogger` API and the fluent API.
- sets the correct caller boundary, when the classic API methods are invoked.

Author: ppkarwasz

slf4j-api/src/main/java/org/slf4j/event/DefaultLoggingEvent.java

@@ -52,7 +52,9 @@ public void addArgument(Object p) {
     }
 
     public void addArguments(Object... args) {
-        getNonNullArguments().addAll(Arrays.asList(args));
+        if (args != null) {
+            getNonNullArguments().addAll(Arrays.asList(args));
+        }
     }
 
     private List<Object> getNonNullArguments() {