Makes singleton variable name of NotificationCenter lowercase

When we require a singleton, its instance name should be named
by lowercase, since it is not a class.

Relates to #128

app/Game/Core/GameObjects/Tile.js

@@ -7,7 +7,7 @@ define([
     "Lib/Utilities/Assert"
 ],
  
-function (Parent, Box2D, Settings, Exception, Nc, Assert) {
+function (Parent, Box2D, Settings, Exception, nc, Assert) {
 
 	"use strict";