get go-dbus compiling on windows

This is for issue #2

rather than making a wrapper, it made more since to add conditional compilation and decouple connections from the unix transport.

I tested building this on my windows 8 machine and OSX 10.8 and also ran go test.

I admit both the original code and this just sits there when I run go test... but I got errors when I broke things and so I think this holds water.

Let me know if you need any other changes.