Fix FileUtil::getUserHomeDir on Mac

getUserHomeDir of PDepend\Util\FileUtil is using stripos(PHP_OS, 'win')
for judgment of Windows system.
But it is not right, because PHP_OS on Mac is 'Darwin'.

% php -r 'var_dump(PHP_OS);'
string(6) "Darwin"
Merge pull request #165 from monmon/master

Fix FileUtil::getUserHomeDir on Mac