[#2023] Add support for readLink.


Problem description
===================

We had some untested readlink support on Linux but it was bad and was only on Windows.

We need an uniform support on all platforms.



Changes description
===================

Add support for readLink.

This also improves support for exists() in Windows as now will check if target exists and have similar behavior on Unix and Windows.

readLink will only read the link at a single level and will not follow links for links.


How to try and test the changes
===============================

reviewers @alibotean 
depends-on: #25 

Check that changes are ok.

This was also developed using TDD and buildbot without touching the Windows system for manual testing. Automated test are the only tests I have performed.

Once we have a basic symlink support, it  will be integrated into server and Laura can perform more brutal tests.