Fix for 'link' issues in Windows

os.path.realpath doesn't follow links in Windows:

http://bugs.python.org/issue9949

This makes 'yotta link' and 'yotta link-target' unusable from Windows.
This patch fixes that by moving 'realpath' in fsutils and providing a
(hopefully) correct implementation for Windows. Tested with
'yotta link-target'.
Merge pull request #101 from bogdanm/fix-win-realpath

Fix for 'link' issues in Windows