Handle Windows path separator when determining config path of descriptio...

I'm making use of your desk-php project, so thanks for that.  It does seem to have a couple problems on Windows though.  This change makes sure that the service description is built properly regardless of directory separator.  Although it looks like there's something else going on with response parsing in desk-php that I haven't been able to track down quite yet, this was the first step to actually be able to access the API commands.

Let me know if you see any issues, and thanks again!