Add request.current_service attribute

[almet]

In #98, @rfk said:

As a larger architectural thought, I wonder if we should pass the service object into each validator by default. IOW, make the signature of a validator function "validate(service, request)" or similar. How common is it to make a validator that closes over the service object like this?

[almet]

That is something I did a lot for this CORS work, and probably it could be useful in other cases. I'm also thinking about making the service available in the view as well (maybe in request.service). What do you think?

[leplatrem]

def current_service(request):
    """Return the Cornice service matching the specified request.

    :returns: the service or None if unmatched.
    :rtype: cornice.Service
    """
    if request.matched_route:
        services = request.registry.cornice_services
        pattern = request.matched_route.pattern
        try:
            service = services[pattern]
        except KeyError:
            return None
        else:
            return service

config.add_request_method(current_service, reify=True)