resolves #58 check that Opal is not already defined on the global object

Author: ggrossetie

src/opal.js

@@ -31,9 +31,9 @@
   if (!('log' in console)) { console.log = function () {}; }
   if (!('warn' in console)) { console.warn = console.log; }
 
-  if (typeof(this.Opal) !== 'undefined') {
+  if (typeof(global_object.Opal) !== 'undefined') {
     console.warn('Opal already loaded. Loading twice can cause troubles, please fix your setup.');
-    return this.Opal;
+    return global_object.Opal;
   }
 
   var nil;