Fix recursion when listing directories

Sftp::scanDirectory() always recursively scans a whole directory tree even if no recursion is wanted. This patch uses scandir instead of the recursive method-call to avoid this.

Because this patch uses scandir() there might be the same problems described in an [issue](https://github.com/Herzult/php-ssh/issues/15) and [pull request](https://github.com/Herzult/php-ssh/pull/19).

This is my first pull request ever, so feel free to point out things I made wrong.