Fix support for except in nameclasses

For https://bugzilla.gnome.org/show_bug.cgi?id=565219

The code was imply missing even if simple, added a few regression
tests.
diff --git a/relaxng.c b/relaxng.c
index 5357b56..5779e7f 100644
--- a/relaxng.c
+++ b/relaxng.c
@@ -3819,7 +3819,11 @@
             return (0);
         return (1);
     } else if (def1->type == XML_RELAXNG_EXCEPT) {
-        TODO ret = 0;
+        ret = xmlRelaxNGCompareNameClasses(def1->content, def2);
+	if (ret == 0)
+	    ret = 1;
+	else if (ret == 1)
+	    ret = 0;
     } else {
         TODO ret = 0;
     }