goBack() will redirect user to his returnUrl
fixes #925

Added Controller::enableCsrfValidation to support turning on/off CSRF validation for particular actions.

This reverts commit 9d7597d6, reversing
changes made to 6b6390f7.

Result of a discussion with @qiangxue
- there is not global Yii::$app->response anymore, every action has
  dedicated response class
- implementation allows using HMVC pattern now, nested actions do not
  interfer

Appended newlines to every PHP document that did not have a newline as the last character in the file.

I18N WIP.