symlink is destroyed after moving from tmp folder by drush.

This code is about: https://github.com/kraftwagen/kraftwagen/issues/22

I just needed it sooner than I had expected and I coded what you said.

Unfortunately, it does not work because `drush` makes the symlink while the build is still on `/tmp`, and later when drush copy the instance to the installation folder, it is not a symlink anymore.

I created the Pull Request just in case you want to check it out.
Any suggestions are welcome.