First Pull request for code review - Registration, User Log in, Pagination

throwing a ValidationException when the user exists is misleading. Please create a different exception for this in app_exception.php. Also, you may use $row as variable name instead of $exist since the output is a row from the database table.
```php
if ($row) {
   throw UserExistsException;
}

$db->insert('user_detail', $values);
```