Add deleteUser method

Author: kjetilk

src/user-store.js

@@ -194,6 +194,20 @@ class UserStore {
       })
       .then(() => this.backend.put('users', userKey, user))
   }
+  /**
+   * Permanently deletes the files of a user
+   *
+   * @param user {UserAccount}
+   *
+   * @return {Promise}
+   */
+
+  deleteUser (user) {
+    let userKey = UserStore.normalizeIdKey(user.id)
+
+    return Promise.resolve()
+      .then(() => this.backend.remove('users', userKey, user))
+  }
 
   /**
    * Saves an "alias" user object, used for linking local account IDs to