[Log] refactor the Log#listenerCount method

Previously the method would conditionally execute one or multiple code paths,
but now the implementation is chosen when the class is initialized and the language
was updated to not specify version numbers.

src/lib/log.js

@@ -62,19 +62,26 @@ Log.prototype.close = function () {
   }
 };
 
-Log.prototype.listenerCount = function (event) {
-  // node >= 3.0 supports EE#listenerCount()
-  if (EventEmitter.prototype.listenerCount) {
-    return EventEmitter.prototype.listenerCount.call(this, event);
-  }
-
-  // compatability for node < 0.10
-  if (EventEmitter.listenerCount) {
+if (EventEmitter.prototype.listenerCount) {
+  // If the event emitter implements it's own listenerCount method
+  // we don't need to (newer nodes do this).
+  Log.prototype.listenerCount = EventEmitter.prototype.listenerCount;
+}
+else if (EventEmitter.listenerCount) {
+  // some versions of node expose EventEmitter::listenerCount
+  // which is more efficient the getting all listeners of a
+  // specific type
+  Log.prototype.listenerCount = function (event) {
     return EventEmitter.listenerCount(this, event);
-  }
-
-  return this.listeners(event).length;
-};
+  };
+}
+else {
+  // all other versions of node expose a #listeners() method, which returns
+  // and array we have to count
+  Log.prototype.listenerCount = function (event) {
+    return this.listeners(event).length;
+  };
+}
 
 /**
  * Levels observed by the loggers, ordered by rank