Tie AccessProvider back to service Provider.

To issue a revokation request, I needed to know the Identity API
endpoint.  This endpoint exists in the Provider used to create the
AccessProvider.  This change threads a link back to the Provider.
diff --git a/reauth_test.go b/reauth_test.go
index c1911e3..62d3e7e 100644
--- a/reauth_test.go
+++ b/reauth_test.go
@@ -107,6 +107,9 @@
 func (my *MyAccess) AuthToken() string {
 	return ""
 }
+func (my *MyAccess) Revoke(string) error {
+	return nil
+}
 
 func TestReauthHandlerUsesSameAccessProvider(t *testing.T) {
 	fakeAccess := &MyAccess{}