Increase compatiblity with sinon.useFakeTimers()

By caching reference to the global setImmediate and setTimeout we can ensure
that promise-polyfill still works as excpected when sinon.useFakeTimers()
replaces them with fake versions.

Promise.js

@@ -1,5 +1,7 @@
 (function(root) {
 
+	var setTimeout = root.setTimeout;
+
 	// Use polyfill for setImmediate for performance gains
 	var asap = (typeof setImmediate === 'function' && setImmediate) ||
 		function(fn) { setTimeout(fn, 1); };
@@ -187,4 +189,4 @@
 		root.Promise = Promise;
 	}
 
-})(this);
+})(global || this);