[DEBUG] More descriptive event reporting

This patch improves DEBUG reporting so that circular event
arguments are shown with some information.

lib/helper.js

@@ -158,19 +158,24 @@ class Helper {
      * @return {string}
      */
     function stringifyArgument(arg) {
-      if (typeof arg !== 'function') {
+      if (Helper.isString(arg) || Helper.isNumber(arg) || !arg)
-        try {
         return JSON.stringify(arg);
-        } catch (e) {
+      if (typeof arg === 'function') {
-          // The object was recursive
-          return arg.toString();
-        }
-      }
         let text = arg.toString().split('\n').map(line => line.trim()).join('');
         if (text.length > 20)
           text = text.substring(0, 20) + '…';
         return `"${text}"`;
       }
+      const state = {};
+      const keys = Object.keys(arg);
+      for (let key of keys) {
+        const value = arg[key];
+        if (Helper.isString(value) || Helper.isNumber(value))
+          state[key] = JSON.stringify(value);
+      }
+      const name = arg.constructor.name === 'Object' ? '' : arg.constructor.name;
+      return name + JSON.stringify(state);
+    }
   }
 
   /**