[fix] fix JSONP transport in Node.js environment

The commit 99bcc62 resulted in `window is not defined` errors.

lib/transports/polling-jsonp.js

@@ -30,6 +30,15 @@ var callbacks;
 
 function empty () { }
 
+/**
+ * Until https://github.com/tc39/proposal-global is shipped.
+ */
+function glob () {
+  return typeof self !== 'undefined' ? self
+      : typeof window !== 'undefined' ? window
+      : typeof global !== 'undefined' ? global : {};
+}
+
 /**
  * JSONP Polling constructor.
  *
@@ -46,8 +55,8 @@ function JSONPPolling (opts) {
   // we do this here (lazily) to avoid unneeded global pollution
   if (!callbacks) {
     // we need to consider multiple engines in the same page
-    if (!window.___eio) window.___eio = [];
-    callbacks = window.___eio;
+    var global = glob();
+    callbacks = global.___eio = (global.___eio || []);
   }
 
   // callback identifier