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

It was mentioned in AssetManager::publish phpdoc.

- rename FileHelper::mkdir for API consistency
- changed default permission for directories to 775 instead of 777
- added some properties to classes that deal with files to allow control
  over directory permissions.

Doc comments at "yii\helpers\base\FileHelper::findFiles()" have been recomposed using markdown format.

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