moved debug fn out to own module, added tests

lib/debug.js

@@ -0,0 +1,15 @@
+"use strict";
+
+module.exports = function(label) {
+  var debug;
+
+  if (process.env.NODE_DEBUG && /\blog4js\b/.test(process.env.NODE_DEBUG)) {
+    debug = function(message) { 
+      console.error('LOG4JS: (%s) %s', label, message); 
+    };
+  } else {
+    debug = function() { };
+  }
+
+  return debug;
+};