use GT_PATH_SEPARATOR and GT_PATH_VAR_SEPARATOR

where necessary
define paths correctly for Windows
coding style
Windows: fopen() fails for directories

define and use gt_file_exists_and_is_dir()
Windows: fix basename unit tests
escape PATH variable
Merge pull request #121 from gordon/windows

Windows: make file path related stuff work

no idea, why travis doesn't show up here for me, but I checked directly on travis and there it shows up and is good.