Add random seed to message name

Two vow can exists in one project - this is just a fact. But it is possible to run both in single millisecond.

lib/vow.js

@@ -60,7 +60,7 @@ var undef,
             }
 
             if(isPostMessageAsync) {
-                var msg = '__promise' + +new Date,
+                var msg = '__promise' + Math.random() + '_' +new Date,
                     onMessage = function(e) {
                         if(e.data === msg) {
                             e.stopPropagation && e.stopPropagation();