Issue title:  exception 'yii\base\InvalidConfigException' with message 'User38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5identityClass must be set.' 
 Issue body: Without    ```
 'user' => [
        'identityClass' => 'app\models\User',
        'enableAutoLogin' => true,
        // You inserted it here
    ],
``` in the web.php configuration file; I am getting error saying: 

> An Error occurred while handling another error: 
> exception 'yii\base\InvalidConfigException' with message 'User38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5identityClass must be set.' 
Is it ok if I add that in the configuration? but I don't understand what that configuration does !!!
 Comments: 
 Comment 0: Did you install Yii2-user using composer?
 Comment 1: no but I have added that line it  working  but I am just worried of the effect.
 Comment 2: You should install Yii2-user using composer and it will work (otherwise I cannot help you in that case)
{'source': 'GithubIssues', 'repo_name_with_owner': 'dektrium/yii2-user', 'issue_id': 154255355, 'repo_stars': 1145.0}