Set serviceWorker to allow offline usage

Close #15, close #14

public/sw.js

@@ -0,0 +1,88 @@
+const debug              = false;
+const CACHE_NAME         = 'csvxml-cache-v1';
+const urlsToCache = [
+
+    '/index.htm',
+    '/assets/js/jszip/dist/jszip.min.js',
+    '/assets/js/csvxmlV2.min.js',
+    '/assets/css/csvxml.min.css',
+
+    '/assets/fonts/SourceSansPro-Regular.woff2',
+
+    '/json/fields.de.json',
+    '/json/fields.en.json',
+    '/json/fields.hu.json',
+
+    '/json/tls.de.json',
+    '/json/tls.en.json',
+    '/json/tls.hu.json',
+
+];
+
+self.addEventListener('install', function(event) {
+    // Perform install steps
+    event.waitUntil(
+        Promise.all([
+        caches.open(CACHE_NAME)
+            .then(function(cache) {
+                if (debug === true) console.log('Opened cache');
+                return cache.addAll(urlsToCache.map(url => new Request(url, {credentials: 'same-origin'})));
+            }),
+
+        ])
+    );
+});
+
+self.addEventListener('fetch', function(event) {
+
+    if (event.request.url.startsWith(location.origin) === false) return;
+
+    event.respondWith(
+        caches.match(event.request)
+            .then(function(response) {
+
+                if (response) {
+                    if (debug === true) console.log("Serving " + event.request.url + " from cache.");
+                    return response;
+                }
+
+                // IMPORTANT: Clone the request. A request is a stream and
+                // can only be consumed once. Since we are consuming this
+                // once by cache and once by the browser for fetch, we need
+                // to clone the response.
+                var fetchRequest = event.request.clone();
+
+                return fetch(fetchRequest).then(
+                    function(response) {
+                        // Check if we received a valid response
+                        if(!response || response.status !== 200 || response.type !== 'basic') {
+                            return response;
+                        }
+
+                        var responseToCache = response.clone();
+
+                        caches.open(CACHE_NAME)
+                            .then(function(cache) {
+                                if (debug === true) console.log("Added " + event.request.url + "to cache");
+                                cache.put(event.request, responseToCache);
+                            });
+
+                        return response;
+                    }
+                ).catch(
+                    () => {
+                        console.log("Problem");
+                        if (event.request.url.indexOf("index.php?") == -1) return;
+                        else {
+                            console.log("Fixing");
+                            return caches.open(CACHE_NAME).then(
+                                function(cache) {
+                                    // cache.keys().then(function(keys) { console.log(keys); })
+                                    // return cache.match("index.php")
+                                });
+                        }
+                    }
+                    );
+            })
+        );
+});