* Fix error with some photos not in any Flickr photoset (album).
* Better error message when the cache directory isn't writable.
* Updated Piwigo version compatibility (no longer support Piwigo less than 2.8).
* Fix error with caching login test that was displaying the login button again after being redirected back to Piwigo.
* Update documentation on the config tab, because we no longer need to set the callback URL.