Map uri template fields to specific responders within the same resource

Add an argument to add_route() to allow responders within the same resource
to be selected based on uri template fields.  This allows multiple
different responders within the same resource to be selected based on the
route defined.

Create a route mapping uri fields like so:

api.add_route('/things/{test}/{foo}', things, map_uri_fields=True)

Create a responder in a resource specific to this route:

def on_get_test_foo(self, req, resp, test, foo):