add(resources, force) {
    let mapped = resources.resources.map(item => {
      let {
        href
      } = item;
      let url = this.resolver(href);
      let encodedUrl = window.encodeURIComponent(url);
      return this.storage.getItem(encodedUrl).then(item => {
        if (!item || force) {
          return this.requester(url, "binary").then(data => {
            return this.storage.setItem(encodedUrl, data);
          });
        } else {
          return item;
        }
      });
    });
    return Promise.all(mapped);
  }
Add all of a book resources to the store
@param  {Resources} resources  book resources
@param  {boolean} [force] force resaving resources
@return {Promise<object>} store objects