Add routers.Resource

This is a convenience class if you want to convert Resource descriptor
classes into routers easily.  Resource descriptor class:

    class MyResource:
        def get(self, request):
            ...
        def post(self, request):
            ...

    router = routers.resource_descriptor_router(MyResource)

routers.Resource gives you a little bit of convenience:

    class MyResource(routers.Resource):
        def get(self, request):
            ...
        def post(self, request):
            ...

    router = MyResource()
Update example.py to use resource-ful utils