Yii2 basic codeception

Yii2-basic boilerplate codeception integration improved. Fixed some entry-points in ```web``` folder. Some things TBD.

TBD:
- [x] Acceptance tests integration and examples
- [x] Functional tests integration and examples
- [x] Unit tests integration and examples
- [x] Standalone extension, which includes: ```TestCase```, ```BasePage``` classes.
- [x] Qiang comment about urls: https://github.com/yiisoft/yii2/pull/1372#issuecomment-29575108. Since we can now refer to Yii::$app in functional and acceptance tests, you can easily create url with ```Yii::$app->urlManager->createUrl($route, $params)```.