Unlink sym-linked folders instead of recursively removing

This PR does a couple of things:

* Instead of recursively removing sym-linked directories, it now just removes the symlink. This is helpful for modules installed via [magento-comoser-installer](https://github.com/magento-hackathon/magento-composer-installer) when using the sym-link deploy method, previous behavior wiped out the files in vendor directory.

* Cover Filesystem with unit-tests
* Fix humanFileSize method name