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

public/js/moment-countdown.js

@@ -1,4 +1,10 @@
+/**
+ * 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.
+ */
 $(document).ready(function() {
+  if (typeof moment !== "undefined") {
     $.each($('.moment-countdown'), function(i, e) {
       var span = $(e);
       var text = span.html();
@@ -8,4 +14,5 @@ $(document).ready(function() {
         span.html(text.replace("%c", timestamp.fromNow()));
       }, 1000);
     });
+  }
 });