add itsdangerous' URLSafeTimedSerializer parser

Parse and return serialized datastructure, if not expired yet. Does not
validate the signature.

    lib.itsdangerous("WzIsIjg1MTM2Yzc5Y2JmOWZlMzZiYjlkMDVkMDYzOWM3MGMyNjVjMThkMzciXQ.BrF35w.IoiMIKcqb2Dgy4Gq7uYXSojcVSM"));

isso/js/app/lib.js

@@ -1,6 +1,7 @@
 define(function (require) {
     return {
         editorify: require("app/lib/editor"),
-        identicons: require("app/lib/identicons")
+        identicons: require("app/lib/identicons"),
+        itsdangerous: require("app/lib/itsdangerous")
     };
 });

isso/js/app/lib/itsdangerous.js

@@ -0,0 +1,51 @@
+define(function() {
+    "use strict";
+
+    // 2011/01/01 in UTC
+    var epoch = 1293840000;
+
+    var decode = function(str) {
+        return atob(str + new Array(str.length % 4 + 1).join("="));
+    };
+
+    var timestamp = function(str) {
+        var bytes = [];
+        for (var i = 0; i < str.length; i++) {
+            bytes.push(str.charCodeAt(i));
+        }
+
+        var a = 0;
+        for (var j = 0; j < bytes.length; j++) {
+            a = a << 8 | +bytes[j];
+        }
+
+        return a + epoch;
+    };
+
+    /*
+     * Load data signed with itsdangerous' URLSafeTimedSerializer.
+     *
+     * If no signature was found or the payload has been expired, return
+     * `null`. Otherwise, return unserialized datastructure.
+     */
+    return function(val, max_age) {
+
+        if (typeof max_age === "undefined") {
+            max_age = 900;
+        }
+
+        var _ = val.split(".", 3),
+            payload = _[0], ts = _[1], signature = _[2];
+
+        if (typeof signature === "undefined") {
+            return null;
+        }
+
+        var age = (new Date()).getTime() / 1000 - timestamp(decode(ts));
+        if (age > max_age) {
+            return null;
+        }
+
+        return JSON.parse(decode(payload));
+    };
+});