Do not fail when localstorage isn't available

Fixes react-stack#1
This is what we do in https://github.com/localForage/localForage/blob/master/src/utils/isLocalStorageValid.js

src/index.js

@@ -2,18 +2,49 @@ function rejectWithMessage(error) {
     return Promise.reject(error.message);
 }
 
+const store = (() => {
+    function isLocalStorageValid() {
+        try {
+            return typeof localStorage !== 'undefined' &&
+                ('setItem' in localStorage) &&
+                typeof localStorage.setItem === 'function';
+        } catch (e) {
+            return false;
+        }
+    }
+
+    if (isLocalStorageValid()) {
+        return localStorage;
+    }
+
+    return {
+      db: {},
+      setItem(key, value) {
+          this.db[key] = value;
+      },
+      getItem(key, value) {
+          const value = this.db[key];
+          if (value === undefined) {
+              // emulate localStorage
+              return null;
+          }
+          return value
+      }
+    };
+})();
+
 export default (key, replacer, reviver) => ({
     load() {
         return new Promise((resolve) => {
-            const jsonState = localStorage.getItem(key);
+            const jsonState = store.getItem(key);
             resolve(JSON.parse(jsonState, reviver) || {});
         }).catch(rejectWithMessage);
     },
 
     save(state) {
         return new Promise((resolve) => {
             const jsonState = JSON.stringify(state, replacer);
-            localStorage.setItem(key, jsonState);
+            store.setItem(key, jsonState);
             resolve();
         }).catch(rejectWithMessage);
     }