add missing prereqs

Thanks CPANTS!
check for git earlier in the test

We could then make Git::Repository an optional prereq, but I'm not sure
if I want to do that... More likely I'll move Statocles::Site::Git and
other git-related modules to their own dist and indicate what to install
in the statocles command script.
fix one of the dir seperator problems on Win32

URLs are always /, so we need to switch the \ on Win32 to /.