Use local var environment rather than NODE_ENV

Traditionally, NODE_ENV refers to an environment variable. For clarity,
let's keep it that way and don't reuse it in the renderer. Also, add a
note about explicitly overriding env vars for node-config.

// FREEBIE

js/background.js

@@ -9,7 +9,7 @@
     };
 
     console.log('background page reloaded');
-    console.log('NODE_ENV', window.config.NODE_ENV);
+    console.log('environment:', window.config.environment);
     extension.notification.init();
 
     var initialLoadComplete = false;