fixes #1993

Moved file and path related methods from StringHelper to FileHelper, renamed StringHelper byte methods not to be misused as string methods

- allows to have dynamic definition of attributes depended on the
  instance
- there was no real need for it to be static. Places that used it static
  have been refactored.

fixes #1318

Model::attributes() has been made static

"yii\sphinx\ActiveQuery" updated to throw exception on conflict between "asArray" and "snippetByModel" options.

yii\sphinx\DataReader removed.