add check for undefined variables in moment-countdown.js and add comments

public/js/moment-countdown.js

@@ -1,13 +1,14 @@
 /**
  * Initialize all moment countdowns on the page. A moment countdown has the
  * class "moment-countdown" and the attribute "data-timestamp" which defines the
- * countdown's goal.
+ * countdown's time goal.
  */
 $(document).ready(function() {
   if (typeof moment !== "undefined") {
-    $.each($('.moment-countdown'), function(i, e) {
+    $.each($(".moment-countdown"), function(i, e) {
       var span = $(e);
       var text = span.html();
+      /* global moment */
       var timestamp = moment(parseInt(span.attr("data-timestamp") * 1000));
       span.html(text.replace("%c", timestamp.fromNow()));
       setInterval(function() {