added isSymbolicLink function for checking symlincs on windows see #34

See  #34.

Fix the `is_link` with a `readlink` workaround
see https://github.com/contao-community-alliance/composer-plugin/issues/34#issuecomment-64711804

> Only Problem: readlink breaks with file usage. so if there is put in a file it shows a warning. The can be resolved with @ or checking if the folder is a folder and not a file ;).

Ok I can move it into a inner if. Is it necessary to check for a `is_dir`?