Added service worker

src/service-worker.js

@@ -0,0 +1,18 @@
+var CACHE = 'v1';
+
+self.addEventListener('fetch', function(event) {
+  event.respondWith(fetchAndCache(event));
+});
+
+function fetchAndCache(event) {
+  return caches.open(CACHE).then(function (cache) {
+    return fetch(event.request)
+      .then(function(response) {
+        cache.put(event.request, response.clone());
+        return response;
+      })
+      .catch(function() {
+        return cache.match(event.request);
+      });
+  });
+}