make dbus-sharp run on Win32

- authentication with Windows user ID was missing
- static Bus constructors didn't work (because of unix: transport), temporarily deactivated them on Win32
- added more robust handling of unix: transport (prevent application termination due to missing libc DLL access)
- polished .csproj a little

I verified that this runs fine with official
http://dbus.freedesktop.org/releases/dbus/dbus-1.7.8.tar.gz

Change-Id: Ie53dc85f970a834789266161a96e3307e6dcf3fd