Add map last updated date to page

js/getMapUpdate.js

@@ -0,0 +1,10 @@
+var mapUpdateTime = document.getElementById('map-last-updated');
+
+function getHeaderTime () {
+  mapUpdateTime.textContent = moment(this.getResponseHeader("Last-Modified")).calendar();
+}
+
+var oReq = new XMLHttpRequest();
+oReq.open("HEAD", "/map/index.html");
+oReq.onload = getHeaderTime;
+oReq.send();