Id site logout

This should handle being able to call the logout endpoint. I've also added a status property to the IdSiteCallbackResult class which basically just mirros what the server send us. This is so the developer can know what kind of callback this is, ie. if it's a logout callback then clear the user's sessions and so on. 

I think the actual handlers for this (ie. on_logout, on_authenticated etc) should be implemented in stormpath-flask and stormpath-django.