fix(native-storage): add missing keys method (#1495)

fix #1415

src/@ionic-native/plugins/native-storage/index.ts

@@ -53,6 +53,13 @@ export class NativeStorage extends IonicNativePlugin {
   @Cordova()
   getItem(reference: string): Promise<any> { return; }
 
+  /**
+   * Retrieving all keys
+   * @returns {Promise<any>}
+   */
+  @Cordova()
+  keys(): Promise<any> { return; }
+
   /**
    * Removes a single stored item
    * @param reference {string}