Add toString() method to HabReceiver.

workspace/ttnhabbridge/src/main/java/nl/sikken/bertrik/hab/habitat/HabReceiver.java

@@ -27,4 +27,9 @@ public final class HabReceiver {
         return location;
     }
 
+    @Override
+    public String toString() {
+        return callSign;
+    }
+    
 }